Low Iodine Diet Research Articles

Effects of iodine repletion on thyroid morphology in iodine and/or selenium deficient rat term fetuses, pups and mothers

It has been suggested that selenium deficiency aggravates the iodine-induced thyroid inflammation and necrosis in iodine-deficient Wistar rats and possibly in man. Studies were carried out to determine whether large amounts of iodine given to iodine-deficient pregnant Sprague-Dawley rats with or without selenium deficiency would induce inflammation and necrosis in their term fetal thyroids. Iodine deficiency was induced in the dams by a low iodine diet or perchlorate in the drinking water and iodine excess was achieved by iodinated drinking water during pregnancy or daily subcutaneous injections of iodine from days 20 to 22 of pregnancy, 1 day after perchlorate was discontinued. Studies were also carried out in 30-day-old pups whose nursing mothers were iodine-deficient (perchlorate) with or without selenium deficiency from conception onward. The administration of iodine restored the morphologic changes in the thyroid induced by iodine deficiency, irrespective of selenium status, toward normal without inflammatory changes or necrosis. Possible explanations for these unexpected findings are discussed.

Dietas com restrição de Iodo (DRI)

No contexto do manuseio do câncer diferenciado de tireóide com 131I, faz-se histórico e descrição das dietas com restrição de iodo (DRI). Elabora-se DRI mais abrangente e auto-explícita. Expõem-se mecanismos de aumento de captação pelas DRI, baseados na expoliação de iodo. Comprova-se a eficacia das DRI em favorecer expoliação e rastreamento; o possível benefício quanto a dose ablativa; não ter-se demonstrado efeito em dose terapêutica, quiçá por insuficiente pesquisa; a importância de obter iodúria inferior a 50ug/dia ou 50ug/g creatinina, para prevenir contaminação e avaliar a eficácia e adesão a DRI. Diante da excessiva ingestão de iodo nos EUA, Reino Unido e países asiáticos, com resultante queda na atividade iodocaptante, deplora-se a escassa adoção de DRI, em relatos de centros de pesquisa, cuja credibilidade só é sustentável, admitindo-se tácita prescrição de DRI informal, raramente constando em "Materiais e Métodos". Outrossim, condena-se o aumento de dose de 131I, na contaminação: a rápida expoliação de iodo por DRI é eficiente e inócua. Comenta-se a inexistência de DRI publicada no Brasil, duvidando-se de sua necessidade, face à deficiência endêmica de iodo, em algumas regiões. O desconhecimento de seu conteúdo, e de eventual excesso, em alguns alimentos, conduz o autor a elaborar algoritmo: 1. Avaliação de quota de iodo alimentar das populações, ou 2. Exclusão de contaminação, pela medida da iodúria, ou 3. Prescrição de DRI. O autor propõe o axioma: "A DRI pode às vezes não ser benéfica, a contaminação é sempre maléfica".

Thyroid Hormone Action on Liver, Heart, and Energy Expenditure in Thyroid Hormone Receptor -Deficient Mice

Thyroid hormone (TH) responsive genes can be both positively and negatively regulated by TH through receptors (TR) α and β expressed in most body tissues. However, their relative roles in the regulation of specific gene expression remain unknown. The TRβ knockout mouse, which lacks both TRβ1 and TRβ2 isoforms, provides a model to examine the role of these receptors in mediating TH action. TRβ deficient (TRβ−/−) mice that show no compensatory increase in TRα, and wild-type (TRβ+/+) mice of the same strain were deprived of TH by feeding them a low iodine diet containing propylthiouracil, and were then treated with supraphysiological doses of L-T3 (0.5, 5.5, and 25 μg/day/mouse). TH deprivation alone increased the serum cholesterol concentration by 25% in TRβ+/+ mice and reduced it paradoxically by 23% in TRβ−/− mice. TH deprivation reduced the serum alkaline phosphatase (AP) concentration by 31% in TRβ+/+ mice but showed no change in the TRβ−/− mice. Treatment with L-T3 (0.5 to 25μ g/mouse/day) caused a 57% decrease in serum cholesterol and a 231% increase in serum AP in the TRβ+/+ mice. The TRβ−/− mice were resistant to the L-T3 induced changes in serum cholesterol and showed increase in AP only with the highest L-T3 dose. Basal heart rate (HR) in TRβ−/− mice was higher than that of TRβ+/+ mice by 11%. HR and energy expenditure (EE) in both TRβ+/+ and TRβ−/− mice showed similar decreases (49 and 46%)and increases (49 and 41%) in response to TH deprivation and L-T3 treatment, respectively. The effect of TH on the accumulation of messenger RNA (mRNA) of TH regulated liver genes was also examined. TH deprivation down regulated spot 14 (S14) mRNA and showed no change in malic enzyme (ME) mRNA in both TRβ+/+ and TRβ−/− mice. In contrast treatment with L-T3 produced an increase in S14 and ME but no change in TRβ−/− mice. From these results, it can be concluded that regulation of HR and EE are independent of TRβ. With the exception of serum cholesterol concentration and liver ME mRNA accumulation, all other markers of TH action examined during TH deprivation exhibited the expected responses in the absence of TRβ. Thus, as previously shown for serum TSH, TRβ is not absolutely necessary for some changes typical of hypothyroidism to occur. In contrast, except for HR and EE, the full manifestation of TH-mediated action required the presence of TRβ.

Thyroid hormone action on liver, heart, and energy expenditure in thyroid hormone receptor beta-deficient mice.

Thyroid hormone (TH) responsive genes can be both positively and negatively regulated by TH through receptors (TR) alpha and beta expressed in most body tissues. However, their relative roles in the regulation of specific gene expression remain unknown. The TR beta knockout mouse, which lacks both TR beta1 and TR beta2 isoforms, provides a model to examine the role of these receptors in mediating TH action. TR beta deficient (TR beta-/-) mice that show no compensatory increase in TR alpha, and wild-type (TR beta+/+) mice of the same strain were deprived of TH by feeding them a low iodine diet containing propylthiouracil, and were then treated with supraphysiological doses of L-T3 (0.5, 5.5, and 25 microg/day/mouse). TH deprivation alone increased the serum cholesterol concentration by 25% in TR beta+/+ mice and reduced it paradoxically by 23% in TR beta-/- mice. TH deprivation reduced the serum alkaline phosphatase (AP) concentration by 31% in TR beta+/+ mice but showed no change in the TR beta-/- mice. Treatment with L-T3 (0.5 to 25 microg/mouse/day) caused a 57% decrease in serum cholesterol and a 231% increase in serum AP in the TR beta+/+ mice. The TR beta-/- mice were resistant to the L-T3 induced changes in serum cholesterol and showed increase in AP only with the highest L-T3 dose. Basal heart rate (HR) in TR beta-/- mice was higher than that of TR beta+/+ mice by 11%. HR and energy expenditure (EE) in both TR beta+/+ and TR beta-/- mice showed similar decreases (49 and 46%) and increases (49 and 41%) in response to TH deprivation and L-T3 treatment, respectively. The effect of TH on the accumulation of messenger RNA (mRNA) of TH regulated liver genes was also examined. TH deprivation down regulated spot 14 (S14) mRNA and showed no change in malic enzyme (ME) mRNA in both TR beta+/+ and TR beta-/- mice. In contrast treatment with L-T3 produced an increase in S14 and ME but no change in TR beta-/- mice. From these results, it can be concluded that regulation of HR and EE are independent of TR beta. With the exception of serum cholesterol concentration and liver ME mRNA accumulation, all other markers of TH action examined during TH deprivation exhibited the expected responses in the absence of TR beta. Thus, as previously shown for serum TSH, TR beta is not absolutely necessary for some changes typical of hypothyroidism to occur. In contrast, except for HR and EE, the full manifestation of TH-mediated action required the presence of TR beta.

The neuropeptides, VIP and NPY, that are present in the thyroid nerves are not released into the thyroid vein.

In the present study, we tested the hypothesis that the neuropeptides, vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY), which are present in the thyroid nerves, act as physiological neurotransmitters involved in the regulation of thyroid hormone secretion and thyroid blood flow. Specifically, we examined whether these neuropeptides can be released into thyroid blood vessels by electrical stimulation of the major thyroid nerves or whether their expression is altered by changes in iodine intake. Sprague-Dawley rats were used in this study. The cervical sympathetic trunk or the superior laryngeal nerve was stimulated by bipolar electrodes in anesthetized rats. During nerve stimulation, blood samples were withdrawn from the thyroid vein. Thyroid blood flow was monitored by laser Doppler blood flowmetry. Sympathetic stimulation caused a marked decrease in thyroid blood flow, which was associated with a significant increase in release of norepinephrine. However, these effects were not accompanied by any change in NPY release into the thyroid vein. Stimulation of the superior laryngeal nerve was not associated with changes in thyroid blood flow or VIP release into the thyroid vein. In a separate experiment, rats were fed a diet containing low-, high-, or normal iodine concentrations. Triiodothyronine (T3) and thyroxine (T4) levels in thyroid venous plasma were significantly reduced in rats fed a low-iodine diet but not in a separate group of rats fed a high iodine diet. However, these treatments had no effect on VIP or NPY concentrations in thyroid venous plasma or in thyroid ganglia. Thus, our results indicate that VIP and NPY, which are present in the thyroid nerves, may not be directly involved in the regulation of thyroid function.

Role of apoptosis of thyrocytes in a rat model of goiter. A possible involvement of Fas system.

Apoptosis, a physiological process of cell death, may modulate the mass of the thyroid gland. We investigated the role of apoptosis and the possible involvement of Fas/Fas ligand (FasL) system in apoptosis during goiter formation and involution in a rat model of goiter. Rats were fed a low iodine diet and a goitrogen, 6-propyl-2-thiouracil, to induce goiter. Rats with goiter were then fed a high iodine diet to study the phase of involution. We examined the presence of apoptosis by electron microscopy (EM) and terminal deoxy-UTP nick end labeling (TUNEL). We also investigated the association between Fas and FasL expression and thyrocyte apoptosis using immunohistochemistry and Western blotting. To evaluate the proliferation of thyrocytes, proliferating cell nuclear antigen was examined immunohistochemically. The number of apoptotic cells increased during goiter formation and the early stage of involution, which were also associated with increased number of Fas-positive thyrocytes, and some of these cells contained TUNEL-positive nuclei. However, the expression of FasL was almost constant throughout the experiment. Proliferating cell nuclear antigen/TUNEL ratio markedly increased during goiter formation but decreased particularly during the late stage of goiter involution. Our results indicate that apoptosis of thyrocytes is a main factor of cell loss during goiter formation and involution and suggest that the Fas/FasL system is involved in the induction of apoptosis of these cells. Moreover, the delicate balance between apoptosis and cell proliferation may play an important role in the control of thyroid gland mass.

Presence and possible role of vascular endothelial growth factor in thyroid cell growth and function.

Angiogenesis is an important component in the development of thyroid goitre. Vascular endothelial growth factor (VEGF) represents a family of specific endothelial cell mitogens involved in normal angiogenesis and in tumour development. The purpose of this study was to determine the distribution of VEGF in thyroid tissues during goitre formation, and to study the actions of VEGF on the regulation of thymidine incorporation and iodine uptake by thyroid follicular cells. Goitre was induced in adult rats by administration of methimazole together with a low iodine diet. Thyroid from normal or goitrous rats was removed, fixed and sectioned. Immunocytochemistry performed for VEGF using the avidin-biotin system showed that VEGF is present in normal thyroid and is located mainly in the vascular endothelium and interfollicular stromal tissue. After administration of goitrogen for 2 weeks, which caused a two- to threefold increase in thyroid weight, staining of VEGF was less apparent within the interfollicular stroma, but strongly increased throughout the thyroid follicular and endothelial cells. Uptake of [125I] and incorporation of [3H]thymidine by Fisher rat thyroid cells (FRTL-5) were measured after 72 h culture with or without TSH or VEGF, or both. In the absence of TSH, incubation with VEGF caused a significant reduction in [3H]thymidine incorporation, but did not significantly alter [125I] uptake. Incubation with TSH (1 mU/ml) caused a fourfold increase in [3H]thymidine incorporation that was diminished by co-incubation with 10 ng/ml or greater VEGF. Similarly, 10 ng/ml or greater VEGF significantly reduced the ability of TSH to increase [125I] uptake. The antagonistic effects of VEGF on TSH-stimulated [3H]thymidine incorporation or [125I] uptake were significantly reduced in the presence of an anti-VEGF antiserum. A DNA fragment representing mRNA encoding the VEGF receptor, flt-1, was identified in FRTL-5 cells by reverse transcription PCR analysis, and the abundance of this fragment was increased in FRTL-5 cells cultured in the medium containing TSH (1 mU/ml) or fibroblast growth factor (FGF)-2 (25 ng/ml). These results indicated that VEGF and one of its receptors, Flt-1, are present in epithelial cells of the thyroid, and that VEGF could contribute to the regulation of development and function of thyroid epithelial cells.

Antigoitrogenic effect of combined supplementation with dl-alpha-tocopherol, ascorbic acid and beta-carotene and of dl-alpha-tocopherol alone in the rat.

The effects of the vitamins dl-alpha-tocopherol, ascorbic acid and beta-carotene, free radical scavengers and lipid peroxidation inhibitors, were analyzed in male Wistar rats made goitrous by feeding a low iodine diet (< 20 micrograms iodine/kg) and perchlorate (1% in drinking water) for 4, 8, 16, and 32 days. Groups of control or goitrous rats received for at least 16 days before killing a diet containing 0.6% vitamin E (as dl-alpha-tocopherol acetate), 1.2% vitamin C (ascorbic acid) and 0.48% beta-carotene, either simultaneously (vitamin cocktail) or separately. This treatment led to a 5-fold increase of vitamin E in the thyroid gland, a 24-fold increase in the liver and a 3-fold increase in the plasma. In control rats, vitamin cocktail administration increased slightly the thyroid weight with little changes in thyroid function parameters. During iodine deficiency, administration of the vitamin cocktail or vitamin E alone reduced significantly the rate of increase in thyroid weight, and DNA and protein contents, as well as the proportion of [3H]thymidine labeled thyroid follicular cells, but not that of labeled endothelial cells. Plasma tri-iodothyronine, thyroxine, TSH levels, thyroid iodine content and concentration as well as relative volumes of glandular compartments were not modified. The proportion of necrotic cells rose from 0.5% in normal animals to about 2% after 16 days of goiter development. No significant protective effect of the vitamins was observed. These results suggest that these vitamins, particularly vitamin E, modulate one of the regulatory cascades involved in the control of thyroid follicular cell growth, without interfering with the proliferation of endothelial cells.

Thyrotropin regulates the levels of free ubiquitin and ubiquitin-protein conjugates in the male mouse thyroid.

The conjugation of ubiquitin to proteins can be a signal for their degradation, but can also be involved in regulatory processes not directly involved in protein degradation. Because thyrotropin (TSH) is the major physiological regulator of the thyroid, we have investigated whether changes in the circulating level of TSH influence the level of immunoreactive ubiquitin in the thyroid, as assessed by dot-blot assays. Putting male Balb/c mice on a low iodine diet with methimazole (MMI) in their drinking water for 14 days raised the level of ubiquitin by 425 % (p < 0.001) per microgram nonthyroglobulin protein (the mean thyroglobulin level dropped by 35% (p < 0.05)). Western blots similarly indicated that immunoreactivity migrating in the region of monoubiquitin, ubiquitin oligomers, and ubiquitinated thyroid proteins increased on the low iodine/MMI diet. Injecting 1 microg triiodothyronine (T3) 6 hours prior to sacrifice appeared to reduce the ubiquitin levels by 31% when compared with mice only on the low iodine/MMI (p < 0.07), but injection of T3 had no effect on ubiquitin levels in mice on the control diet. Injecting male ICR mice with a large dose of TSH (200 mU) increased immunoreactive ubiquitin levels by 50% (p < 0.05) 2 hours later. After a second dose of TSH was injected 12 hours later, the level of immunoreactive thyroglobulin fell by 17% (p < 0.05). With further 12 hourly injections, thyroglobulin levels then began to reaccumulate, and they had returned to the level of saline-injected controls after 50 hours (five TSH injections), while ubiquitin levels fell, but remained significantly elevated above the saline-injected controls (36%, p < 0.01). When ICR mice were given perchlorate in their drinking water to block the iodide pump, thus preserving thyroidal responsiveness to repeated TSH injections, the responses to the initial two TSH injections were similar to mice who received ordinary tap water. However, with further TSH injections, the reaccumulation of thyroglobulin did not occur: at 50 hours, thyroglobulin levels remained suppressed by 28% (p < 0.05), and ubiquitin levels actually rose slightly, and were significantly higher (57%, p < 0.01) than the saline-injected controls. These in vivo responses of free ubiquitin, oligoubiquitin, and ubiquitinated protein to changes in the level of circulating TSH suggest that ubiquitin-mediated mechanisms are involved in some of the thyroid's metabolic responses.

Thyrotropin regulation by thyroid hormone in thyroid hormone receptor beta-deficient mice.

Thyroid hormone responsive genes can be both positively and negatively regulated by thyroid hormone. TSH is down-regulated by thyroid hormone and rises during thyroid hormone deprivation. Because both thyroid hormone receptor (TR) alpha and beta genes are expressed in the pituitary gland, it is unclear what the relative roles of TR alpha and TR beta are in TSH regulation. Experiments using over expression of artificial genes have yielded conflicting results. The TR beta knock-out mouse that lacks both TR beta1 and TR beta2 isoforms provides a model to examine the role of these receptors in TSH regulation. TR beta deficient (TR beta-/-) and wild-type (TR beta+/+) mice of the same strain were deprived of thyroid hormone by feeding them a low iodine diet containing propylthiouracil and were then treated with different doses of L-T3 and L-T4. Thyroid hormone deprivation rapidly increased the serum TSH level in both TR beta+/+ and TR beta-/- mice, reaching a similar level in the absence of thyroid hormone. In contrast, the decline of serum TSH by treatment with both L-T3 and L-T4 was severely blunted in TR beta-/- mice, and full suppression was not achieved with the maximal L-T3 dose of 25 microg/day x mouse. These data indicate that TR beta is not required for the up-regulation of TSH in thyroid hormone deficiency. However, although TR alpha alone can mediate thyroid hormone induced TSH suppression, TR beta enhances the sensitivity of TSH down-regulation and may be essential for the complete suppression of TSH.

Myelin basic protein immunoreactivity in the internal capsule of neonates from rats on a low iodine intake or on methylmercaptoimidazole (MMI)

Rats fed on low iodine diets (LIDs) result in a normal circulating level of triiodothyronine (T3), a low level of thyroxine (T4) and an elevated thyroid-stimulating hormone (TSH). These changes are similar to those observed in habitants who live in iodine-deficient areas and different from those observed when the hypothyroidism is produced by goitrogens. To study the effects of LID or goitrogens on the myelin basic protein (MBP) immunoreactivity (MBP-ir) during the myelination of the internal capsule, one group of experimental female rats was fed on an LID, and another group received a standard laboratory diet with methylmercaptoimidazole (MMI) added in the drinking water. Animals fed on a standard laboratory diet and animals fed on an LID supplemented with KI were used as controls. At P10, the MMI treatment has produced a more marked decrease in the surface density of MBP-ir processes with respect to controls than that produced in the LID animals. This decrease was correlated with the cerebral concentrations of triiodothyronine (T3) we found. During the postnatal development, a recovery in the levels of the surface density with respect to controls was observed in both experimental groups. The recovery occurred by P20 in the LID group and by P32 in the MMI rats.

Effects of maternal iodine deficiency and thyroidectomy on basal neuroendocrine function in rat pups.

We have used in situ hybridization histochemistry to investigate the effects of maternal thyroidectomy and chronic maternal iodine deficiency on basal neuroendocrine function in rat pups. Specifically, we have measured hypothalamic thyrotrophin-releasing hormone (TRH) and pituitary thyroid-stimulating hormone (TSH) expression together with circulating levels of tri-iodothyronine (T3) in rat pups delivered from and suckled by thyroidectomized or iodine-deficient dams. Because of the close interaction between the thyroid, adrenal and growth hormone axes, we have also examined hypothalamic corticotrophin-releasing hormone (CRH) and growth hormone-releasing hormone (GRH) transcripts at the same time points: birth, 1 month and 2 months of age. Three weeks after surgical thyroidectomy, adult female Sprague-Dawley rats proved unable to carry pups to term and lactate successfully. Pups delivered from thyroidectomized dams given a small replacement dose of T3 during pregnancy were significantly lighter than controls (84 +/- 3%) and had markedly depressed plasma T3 levels (36 +/- 6% of control). Hypothalamic CRH and GRH transcript levels were significantly decreased in pups at birth (to 8 +/- 2.5% and 24 +/- 8% of control respectively) but had returned to normal by 1 month after delivery. Pituitary TSH transcript levels and hypothalamic levels of TRH transcripts, however, were similar to those of controls. Only one of seven dams fed a low-iodine diet for 6 months produced live pups, and these were too few in number to produce significant data. Dams fed a low-iodine diet from 4 months before mating, however, did produce live pups and although they were not significantly lighter than control pups at birth, by 1 month after birth, they were significantly lighter (72 +/- 3% of controls). Circulating T3 levels were not significantly different from control at any time point examined. Hypothalamic TRH levels were significantly elevated at birth (451 +/- 138% of control), but this difference was not maintained at 1 or 2 months after birth despite the lactating dams being maintained on the low-iodine diet. Pituitary TSH levels showed an upward trend at all time points that reached significance at 1 month after birth (204 +/- 19%; P < 0.05). Hypothalamic CRH and GRH transcript levels were not different from controls at any time point. In summary, chronic iodine deficiency or thyroidectomy with low-level T3 replacement in Sprague-Dawley rats markedly impaired fertility and the ability to carry pups to term, and produced an unexpectedly modest up-regulation of the hypothalamo-pituitary-thyroid axis and down-regulation of the hypothalamo-pituitary-adrenal axis.

Hypothyroidism alters the effect of GTP on adenylyl cyclase in forebrain and hindbrain synaptosomal membranes from 15-day-old rats

The effect of GTP concentration on forskolin-stimulated adenylyl cyclase activity was examined in synaptosomal membranes from 15-day-old rats that were hypothyroid due to administration of propylthiouracil and a low-iodine diet to the mothers during pregnancy and suckling. In membranes from the forebrain hypothyroidism abolished the overall stimulatory effect of GTP, which was seen in the euthyroid case. In membranes from the hindbrain hypothyroidism had the opposite effect in that there was an enhancement of an overall stimulatory effect of GTP. It is suggested that these findings reflect changes during early development of the brain in the expression of various G-proteins and/or the expression of different isoforms of adenylyl cyclase.

Changes in the immunohistochemical localisation of fibroblast growth factor-2, transforming growth factor-beta 1 and thrombospondin-1 are associated with early angiogenic events in the hyperplastic rat thyroid.

Administration of a goitrogen (methimazole) and a low iodine diet to rats over a two-week period resulted in hypothyroidism and thyroid hyperplasia compared with controls (control: total serum thyroxine (T4) 66 +/- 4 nmol/l, thyroid weight 5 +/- 1 mg/100 g body weight; experimental: T4 undetectable, thyroid weight 27 +/- 4 mg/100 g body weight after 2 weeks of treatment; mean +/- S.D., n = 10). Immunohistochemistry carried out using a specific endothelial cell marker, CD31, and morphometric analysis (point counting of immunopositive cells) revealed that the progression of goitre in the rat thyroid is accompanied by an increase in capillary endothelial cell growth (neovascularisation). Fibroblast growth factor-2 (FGF-2) immunohistochemistry revealed widespread staining for the protein in the follicular cells of control glands. Less intense staining was found in the stroma and follicular cell nuclei. During hyperplasia and subsequent neovascularisation there was a progressive increase in the FGF-2 immunoreactivity at all locations during the two-week treatment period. Thrombospondin-1 (TSP1) immunoreactivity in the control rat thyroid was found in the stroma and in the endothelial cells, while weak follicular cell staining was also present. In the goitrous rat thyroid the TSP immunoreactivity was present after 1 week of treatment in the endothelial cells and most follicular cells, whilst stroma localisation was weak. After week 2 of treatment the endothelial cell and stromal localisation was no longer apparent, although a follicular localisation was still present. Transforming growth factor-beta 1 (TGF beta 1) immunoreactivity was present in the cytoplasm of a minority of the follicular cells in control rat thyroids, while their nuclei were unstained. In the goitrous rat thyroid an increase intensity of staining for TGF beta 1 was seen in all follicular cells, many of which now also demonstrated immuno-positive nuclei, within one week of goitrogen administration. These results show that in the hyperplastic thyroid increases in FGF-2 and TGF beta 1, and decreases in TSP1 accompany angiogenesis. These factors may interact in an autocrine/paracrine relationship to stimulate the neovascularisation that occurs during goitre formation.

Effect of Dietary Crude Chitin on Thyroid Function in Chicks Fed a Low Iodine Diet.

Two experiments were conducted with 1-day-old chicks to determine the effect of dietary crude chitin processed industrially from marine crab shell waste on thyroid function in chicks fed a low iodine diet. In Experiment 1, chicks were assigned to four different dietary treatment-groups and fed on each diet for 3 weeks. All diets were formulated by supplementing 5% crude chitin and/or 0.5mg/kg KI to the low iodine basal diet. Dietary treatments in Experiment 2 were similar to those in Experiment 1 except that the low iodine basal diet was supplemented with 0.03% thiouracil (TU). In Experiment 1, the dietary crude chitin or KI ameliorated the decrease in plasma thyroxine concentration, the enlargement of the thyroid gland and the excessive abdominal fat deposition induced by the low iodine basal diet, but crude chitin had no additional effect on these parameters in chicks fed the KI-added diet. In Experiment 2, the dietary crude chitin or KI improved the growth retardation and excessive abdominal fat deposition induced by the low iodine basal diet containing 0.03% TU, but there were no significant differences in the growth and abdominal fat weight among dietary crude chitin, KI and crude chitin+KI treatments. These results suggested that dietary crude chitin improved a low thyroid function induced by the low iodine diet with or without TU in chicks.

Promotion of Thyroid Tumors in F344 Male Rats Given a Low Iodine Diet after Treatment with N-Methyl-NNitrosourea in Their Drinking Water.

Promotion of Thyroid Tumors in F344 Male Rats Given a Low Iodine Diet after Treatment with N-Methyl-NNitrosourea in Their Drinking Water.

Expression of nitric oxide synthase isoforms in the thyroid gland: evidence for a role of nitric oxide in vascular control during goiter formation.

The thyroid gland is a highly vascular tissue, and its blood flow changes dramatically in various pathological conditions. Although the mechanisms regulating these changes in vascularity and blood flow are not well understood, candidate mediators include endothelin-1 (ET-1) and nitric oxide (NO). In the present study, we used a reverse transcriptase-polymerase chain reaction assay to determine which components of these vasoregulatory pathways are present in the thyroid and to analyze changes in gene expression in an experimental model of goiter formation and involution. Expression of messenger RNAs (mRNAs) encoding ET-1, ET receptors (ETA and ETB), ET-converting enzyme, and the three nitric oxide synthase (NOS) isoforms (NOS I, NOS II, and NOS III) was readily detected in the rat thyroid. After goiter formation was induced by thiouracil and a low iodine diet, there was increased expression of the genes encoding ET-related proteins (ET-1, 3.2-fold; ETA, 2.9-fold; ETB, 3.5-fold) as well as two of the three NOS isoforms (NOS I, 2.7-fold; NOS III, 4.9-fold). During iodide-induced involution, the ET-related mRNA levels remained elevated, whereas those of the two NOS isoforms returned to basal values. ET-converting enzyme, NOS II, and thyroglobulin mRNAs were minimally affected in this model, providing evidence for selective regulation of these genes. To assess whether NO plays a role in vascular changes during goiter formation, animals were treated with a NOS inhibitor, N-nitro-L-arginine methyl ester (NAME). NOS activity in the thyroid was inhibited by more than 75% after treatment with NAME. Thyroid hormone and TSH levels were unchanged. Although NAME had little effect on overall thyroid size, vascular expansion during goiter formation was decreased by 36%. We conclude that the thyroid gland expresses a complex network of vasoactive genes whose expression is regulated dynamically during thyroid goiter formation and involution. NO production and probably other locally produced vasoactive substances are involved in changes in thyroid vascularization.

Effect of humic acids on thyroidal function

Humic substances (HS) have been implicated as environmental goitrogens. Increased prevalence of goiter has been recently noticed in the blackfoot disease endemic area on the southwest coast of Taiwan, where well water is rich in HS. This study investigated the in vivo effects of humic acids (HA) on the thyroid gland of rats and mice. Groups of mice and rats were fed regular or moderately iodine deficient (approximately 167 vs 700 micrograms l- per kg) chow and distilled water or HA water (1mg/ml) for 3 or 4 months. Serum T4, T3, reverse T3, and/or TSH were measured by radioimmunoassay. Thyroidal 125I uptake was measured in mice at 2 h after injection of 1 microCi125I ip. Treatment of the rat with HA was associated with a significantly (p < 0.05) reduced serum T4 without a change in other parameters of study. Treatment with low iodine diet was associated with a clear increase in serum T3 and a decrease in serum rT3. Rats treated with both HA and low iodine diet showed a significantly reduced serum T4, increased serum T3 and decreased serum rT3. In mice, treatment with low iodine diet significantly increased thyroidal 125I uptake and additional treatment with HA significantly enhanced the effect of low iodine diet. Treatment with HA did not influence thyroid weight of rats or mice given normal or iodine deficient diets. We conclude that HA per se do not induce goiter, but they may enhance the goitrogenic effect of low iodine.

Expression of the endothelin-1 gene in the rat thyroid gland and changes in its peptide and mRNA levels in goiter formation and iodide-induced involution.

Endothelin-1 (ET-1) is a major vasoconstrictor peptide, first found in endothelial cells, and later in many other tissues, including the thyroid gland. We analysed the expression of the ET-1 gene in the rat thyroid gland and changes in ET-1 mRNA and peptide levels in goiter development and involution, two circumstances characterised by vascular changes. Thyroid hyperplasia was induced in adult Wistar rats by feeding a low iodine diet (LID) supplemented with 0.25% thiouracil for 10 days, and LID alone for 2 further days (H.12 group). Involution was induced by injecting 100 micrograms iodide and refeeding a normal diet during 6 h, 12 h, and 24 h (I.6h, I.12h, I.24h groups). Rats fed a normal iodine diet were used as controls. A specific 488 bp cDNA corresponding to the known sequence of pre-pro ET-1 was found by RT-PCR from RNA extracts in all thyroid experimental groups, as well as in lung and kidney which were used as positive controls. RP-HPLC analysis showed that ET-1 immunoreactivity eluted similarly as mature ET-1. During hyperplasia, ET-1 mRNA and peptide levels were increased 3.5- and 5-fold respectively. The relative volume of the vascular bed was more than doubled. During iodide-induced involution, the glandular ET-1 mRNA level remained elevated. The concentration of ET-1 peptide increased and was significantly greater at 12 h involution than in the H.12 group. At this time, the capillary reticulum reverted to individual capillaries and the vascular bed was significantly reduced. These data demonstrate that the ET-1 gene is expressed in the rat thyroid gland and that the ET-1 mRNA and peptide levels are increased during thyroid hyperplasia and remain elevated during a phase of rapid iodide-induced involution. These data suggest that changes in ET-1 production may play a role in control of thyroid gland trophic regulation and vascularity.

Increase of basic fibroblast growth factor (FGF) and FGF receptor messenger RNA during rat thyroid hyperplasia: temporal changes and cellular distribution

Goitre was induced in adult rats by acute (1 or 2 weeks) or chronic (4 or 10 weeks) administration of methimazole together with a low iodine diet. Involution of thyroid growth was then observed at 16 weeks, 4 weeks after withdrawal of goitrogens and reversion to a normal diet. Experimental animals quickly became hypothyroid compared with controls and exhibited thyroid hyperplasia (control (n = 10): total serum thyroxine (T4) 66 +/- 4 nmol/l, thyroid weight 5 +/- 1 mg/100 g body weight, means +/- S.D.; experimental (n = 10): T4 undetectable, thyroid weight 27 +/- 4 mg/100 g body weight after 2 weeks of treatment). Thyroid growth rate subsequently slowed between 2 and 10 weeks. Messenger RNA for basic fibroblast growth factor (basic FGF) and for the high-affinity FGF receptor, was compared in the thyroids and livers of control and goitrous rats by ribonuclease protection assay. Low levels of mRNA for basic FGF and its receptor were detectable in thyroids from control rats at all times, while none was detected in the livers from any animal. Basic FGF and receptor mRNAs increased, and were detected at greatest abundance in hyperplastic thyroids at 1 and 2 weeks respectively, during goitre formation, but subsequently declined in parallel with thyroid growth rate at 4 and 10 weeks. When quantified by radioimmunoassay, basic FGF extracted from thyroids was fivefold greater than in controls after 1 week of goitrogen treatment (control (n = 4): 24 +/- 9 pmol/micrograms DNA; goitre (n = 4): 100 +/- 16 pmol/micrograms DNA; P < 0.05). Basic FGF and FGF receptor mRNAs localized by in situ hybridization predominantly to the epithelial cell population within follicles. Localization by immunohistochemistry demonstrated that basic FGF was present in the thyroids of control rats, and was largely associated with the basement membrane of follicles. During thyroid hyperplasia, increased basic FGF immunoreactivity appeared over the cytoplasm of follicular epithelial cells and was lost from the extracellular matrix. Thyroid involution following removal of goitrogen/low iodine treatment was associated with a decrease in mRNA for basic FGF or its receptor, and a loss of immunoreactive basic FGF from the cytoplasm of follicular cells. These results suggest that autocrine expression of basic FGF and FGF receptor could contribute to thyroid hyperplasia in rats.