Low Iodine Diet Research Articles

Effects of pentachlorophenol (PCP) on the pituitary and thyroidal hormone regulation in the rat

Investigations on rats after repeated application of pentachlorophenol (PCP) should clarify whether thyroid side effects caused by this xenobiotic can be seen in a dose range which does not cause major toxic effects. Female rats of the Wistar strain were treated with 3 and 30 mg PCP/kg body wt. daily by gavage for 28 days. To assess the potential impact of impurities the study was performed both with pure and technical grade PCP at the dosage of 3 mg · kg −1 · day −1, and at a level of 30 mg · kg −1· day −1 with pure PCP only. The effects in animals on normal iodine diet were compared to those in animals on a low iodine diet. No decrease of body wt. was noticed, only the liver weights were increased slightly in animals treated with 30 mg · kg −1· day −1 PCP. However distinct effects on thyroid hormones as well as on thyrotropin (TSH) were observed: a pronounced fall of circulating thyroxine (T 4) and triiodothyronine (T 3) levels was accompanied by lower levels of both free thyroid hormones and TSH, and the T 4:T 3 ratio was decreased in serum. Furthermore the intrathyroidal hormone stores were reduced. An interference of PCP at pituitary or hypothalamic level is assumed as a major mode of action. Additional effects of the test substance on peripheral conversion can be suspected.

Serum free thyroxine levels in cats maintained on diets relatively high or low in iodine

Excessive or deficient intake of iodine may play a role in the development of goitre and hyperthyroidism in cats. Previous investigations have shown that the serum free thyroxine level of cats is affected by brief administration of food high or low in iodine content. We have now measured serum free thyroxine levels in groups of cats fed relatively high or low iodine diets for much longer periods (5 months). In contrast to our earlier findings, the chronic ingestion of relatively high or low iodine diets did not lead to statistically significant differences in serum free thyroxine levels. The results of the present investigations show that the cat is able to maintain normal levels of thyroid hormone despite prolonged high or low iodine diets. It may be that the adaptive mechanisms that are called into play during chronic high or low iodine intake lead to thyroid disease, particularly in certain predisposed individuals.

Altered expression of insulin-like growth factor-I (IGF-I) and IGF binding proteins during rat thyroid hyperplasia and involution.

We have investigated changes in the synthesis and localization of insulin-like growth factor (IGF)-I and IGF binding proteins (IGFBPs) in thyroid tissues during the induction of goitre in iodine-deficient rats, and during the subsequent involution of the gland following goitrogen withdrawal. 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. After twelve weeks the goitrogenic stimuli were removed and thyroids examined 4 weeks later. Circulating T4 levels became undetectable within two weeks of goitrogen administration while thyroid weight had increased five-fold. The thyroids continued to increase in size up to 10 weeks, but at a slower growth rate. IGF-I mRNA, detected by ribonuclease protection assay, was present in the control rat thyroid and increased in abundance after both 1 and 2 weeks of goitrogen administration. Levels of IGF-I mRNA showed a relative decline with prolonged goitrogen administration, and following thyroid involution the hybridization signal was similar to that seen in control glands. Northern blot hybridization showed that IGFBP-2, -3 and -5 mRNAs were all present in growth-quiescent, control thyroids and those encoding IGFBP-2 and -3 were elevated in the goitrous glands and remained so as long as goitrogen was administered, thereafter declining during thyroid involution. IGF-I and IGFBP-2 and -3 mRNAs and synthesized peptides, detected by in situ hybridization and immunohistochemistry respectively, were found to co-localize predominantly in follicular epithelial cells. IGFBP-5 mRNA abundance was unaltered during goitre formation, but was increased in the involuting thyroid. Both IGFBP-5 mRNA and peptide were localized to the parafollicular cells (C-cells) which were increased in number during involution. The results suggest that an increased expression of IGF-1 may contribute to early goitre formation, but that a relative increase in the abundance of IGFBP-2 and -3 may limit IGF availability at later times, and facilitate a slowing of thyroid growth rate. The discrete expression of IGFBP-5 by C-cells suggests that it could contribute indirectly to goitre formation or involution by acting in a paracrine fashion.

Iodine deficiency induces thyroid autoimmune reactivity in Wistar rats.

The last 2 decades it has become clear that iodine deficiency has a modulating effect on the thyroid autoimmune response in humans. Also, in animals that spontaneously develop autoimmune thyroid disease, evidence is accumulating that a low iodine intake can modulate thyroid autoimmune reactivity. However, it is still not clear what the effect of a low iodine intake on thyroid autoimmune reactivity is in normal nonautoimmune animals. To study the relationship of a dietary low iodine intake on the thyroid autoimmune reactivity in nonautoimmune animals, normal Wistar rats (female) were kept on an enriched iodine diet (daily iodine intake of 100 micrograms iodine), a "for our area normal" (conventional) diet (COD; daily iodine intake of 7 micrograms iodine), a low iodine diet (LID; 2 days of 1% KCLO4, followed by iodine-deficient drinking water/pellets), or an extremely low iodine diet (LID+; 1% KCLO4 continuously in the drinking water and iodine-deficient pellets). The enriched iodine diet rats were euthyroid (T3, approximately 8 nM/liter: T4, approximately 50 nM/liter; TSH, approximately 2 ng/ml), had a normal thyroid weight (approximately 12.5 mg), and showed only minimal signs of local thyroid immune reactivity; low numbers of intrathyroidal dendritic cells (DC; approximately 35 DC/mm2), CD4+ cells (approximately 2 cells/mm2), and CD8+ cells (approximately 2.5 cells/mm2) were found in combination with low anticolloid antibody production (incidence of positive animals, 12.5%). The COD resulted in a normal thyroid function. The rats were euthyroid (range of T3, 1.6-1.2 nM/liter; T4, approximately 50 nM/liter; TSH, approximately 2 ng/ml) and had a normal thyroid weight (approximately 12.5 mg). However, some signs of thyroid autoimmune reactivity were found [number of intrathyroidal DC, approximately 40/mm2; approximately 3 CD4-positive (CD+) cells/mm2; approximately 3 CD8+ cells/mm2; together with a 30% incidence of anticolloid antibodies]. The LID and LID+ not only induced goiter formation [thyroid weight, 27.3 +/- 4.2 mg (mean +/- SD) after 12 weeks of LID and 38.4 +/- 5.3 mg after 4 weeks of LID+] and low production of T4 by the thyroid [28 +/- 3 nM/liter (mean +/- SD)] after 12 weeks of LID and 14 +/- 3 nM/liter after 2 weeks of LID+], but also induced various signs of thyroid autoimmune reactivity.(ABSTRACT TRUNCATED AT 400 WORDS)

5′ Deiodinase activity in brain regions of adult rats: Modifications in different situations of experimental hypothyroidism

In the central nervous system, type II 5′ deiodinase (5′D-II) is highly regulated, as judged by the dramatic changes in enzyme levels observed after abrupt alterations in thyroid status. In this work, the 5'-DII activity has been studied in different situations of experimental hypothyroidism (propylthiouracil, methimazole, thyroidectomy, and low iodine diet), in various brain regions (pituitary, cerebellum, brain stem, hypothalamus, cortex, and whole brain) in adult rats. Propylthiouracil and methimazole significantly increase the activity in all brain regions. These increases are higher in rats treated with methimazole. Thyroidectomy significantly increases the activity in cortex and pituitary. A low iodine diet significantly increases in all brain regions except in the hypothalamus. The concentration of triiodothyronine (T3) studied in the major brain regions remained unchanged. The results obtained show a compensatory mechanism in pituitary and other brain regions in order to maintain the T 3 levels in brain tissue.

The effects of goitrogenesis, involution, and goitrogenic rechallenge on the clonogenic cell content of the rat thyroid.

A fraction of enzymatically monodispersed rat thyrocytes from untreated animals clonally proliferate into thyroid follicular units following transplantation into the subcutaneous fat pads of syngeneic recipients. During the induction of experimental goiters in rats either with 3-amino-1,2,4-triazole/iodine sufficient diet or KClO4/Remington low iodine diet, the clonogenic fractions of cells from aminotriazole goiters decreased to 1.9 x 10(-4) and KClO4 goiters to 9.8 x 10(-5) as compared to 5.8 x 10(-3) for cells from age-matched controls during the growth phase of goitrogenesis. With continued aminotriazole treatment after thyroid hyperplasia had ceased, the clonogenic fraction increased to 2.0 x 10(-3) while continued KClO4 treatment had little further effect. The changes in the clonogenic fraction induced by both regimens were reversed during involution; goitrogenic rechallenge of involuted glands led to changes in the clonogenic fraction similar to that noted during the initial challenge. The clonogenic fractions of cells from aminotriazole goiters were greater than that of cells from KClO4 goiters at all time points examined despite similar TSH levels in situ. We conclude that the rat thyroid contains a hierarchy of cells with different proliferative capacities and that the clonogenic thyrocytes possess many of the attributes of a stem-cell.

Increase in estrogen receptor levels in MNU-induced thyroid tumors in LE rats.

Estrogen receptor (ER) levels were evaluated in thyroid tumors induced by N-methyl-N-nitrosourea (MNU) and low iodine diet (LID) or propylthiouracil (PTU) in intact and estrogen (E2) loaded Long-Evans (LE) rats. MNU at 40 mg/kg body wt was injected in 50 day-old LE rats of both sexes. The animals were killed 17-22 weeks later and the thyroid tissues were subjected to ER assay. In LID-treated groups, cytosolic ER (cER) levels were 6.7 +/- 5.8 (fmol/mg protein, mean +/- SE) in females and 0.7 +/- 1.4 in males, E2 increased the ER levels. In E2-loaded LID groups, cER levels were 12.9 +/- 3.7 in females and 1.7 +/- 1.7 in males. PTU treatment produced almost comparable ER levels as LID treatment. PTU treatment as well as LID treatment increased the serum TSH levels with E2 treatment producing additional elevation. In evaluating ER levels by histological type of thyroid tumors, the level in cER plus nER showed the lowest value of 6 +/- 6.4 (fmol/mg DNA, mean +/- SE) in hyperplasia, followed by 129 +/- 52.3 in adenoma and 289 +/- 51.7 in carcinoma. The rates of BrdU incorporation in thyroid follicles indicated higher proliferation activity in the area of adenoma and carcinoma rather than in the hyperplastic area. These data suggested that E2 treatment increases the ER levels in MNU and LID/PTU-induced thyroid tumors. The level of ER was correlated to the histological type of thyroid tumors.

The plateau phase rat goiter contains a sub-population of TSH-responsive follicular cells capable of proliferation following transplantation

In the rat, chronic TSH stimulation leads to self-limited thyroid hyperplasia, goitrogenesis, and TSH-responsive thyroid tumors. The current studies were aimed at clarifying the mechanism by which hormone-responsive, proliferating follicular cells arise in quiescent plateau phase rat goiters. Enzymatically monodispersed rat thyrocytes from early plateau phase and involuting goiters were analyzed for the capacity to form thyroid follicular units after transplantation into syngeneic recipients. Goiters induced with two different regimens contained substantial numbers of cells capable of proliferating into functioning thyroid follicular tissue after transplantation. The clonogenic fractions of cells from goiters induced by 3-amino-1,2,4-triazole or KClO4/Remington low iodine diet were significantly lower than that of cells from control glands. Furthermore, the clonogenic fraction of cells from the KClO4 goiters was also found to be significantly less than that of cells from aminotriazole goiters despite similar TSH levels in the cell donors. The hormone responsiveness of the clonogenic goiter cells and the histological appearance and functional capacity of the thyroid tissue which arose after grafting were indistinguishable from controls. We conclude that the capacity to clonally proliferate into follicular units is a specific trait which characterizes a unique subset of follicular cells and speculate that the hormone-responsive tumors which typically appear in the chronically stimulated rat thyroid originate from cells within this subset.

Vasoactive intestinal peptide enhances thyroidal iodide uptake during dietary iodine deficiency.

The presence of vasoactive intestinal peptide and neuropeptide Y in thyroid nerves and their effects on thyroid blood flow are well known. However, the effects of these two neuropeptides on the various processes involved in thyroid hormone biosynthesis and release have not been fully explored. We have now tested these two peptides for effects on an early step in thyroid hormone biosynthesis, namely iodide uptake, a process which is comprised of trapping and organification. In these experiments, we have used anesthetized adult male rats pretreated with thyroxine or fed a low iodine diet to increase thyroidal sensitivity. Vasoactive intestinal peptide significantly increased iodide uptake in rats fed an iodine deficient diet but not in those fed a normal iodine diet. This effect disappeared if animals were pretreated with propylthiouracil. Neuropeptide Y did not alter iodide uptake in rats on either the low or the high iodine diet, regardless of the presence or absence of propylthiouracil. The effect of vasoactive intestinal peptide on iodide uptake could be due to its influence on the organification of iodine, or on thyroid blood flow, or on both processes.

Iodine content of rat thyroglobulin affects its antigenicity in inducing lymphocytic thyroiditis in the BB/Wor rat.

The BB/Wor rat develops spontaneous insulin dependent diabetes mellitus (DM) and lymphocytic thyroiditis (LT). We have recently demonstrated that immunization of BB/Wor rats with allogeneic thyroglobulin (Tg) induces LT at an early age. The incidence of spontaneous and Tg induced LT is extremely variable among different BB/Wor sublines. It has been shown that high iodine diet significantly increases the incidence of spontaneous lymphocytic thyroiditis (LT) and low iodine diet significantly decreases the incidence of LT in genetically predisposed BB/Wor rats. Recent studies on thyroglobulin (Tg) induced LT in chicken and mouse have shown that iodine rich Tg is far more antigenic than Tg with a low iodine content, suggesting that a high iodine diet increases the immunogenicity of Tg molecule. In order to determine whether the extent of Tg iodination would affect its immunogenicity in the BB/Wor rats, the current study was carried out. Normal iodine Tg (NTg) or low iodine Tg (LTg) was obtained from thyroids of rats that were placed on regular diet or regular diet plus 0.5% methimazole, respectively. 120 rats from the NB (highly susceptible) and BB (low susceptible) sublines were randomized in three groups. Immunization was carried out with a 1:1 emulsion of complete Freund's adjuvant (CFA) and LTg, NTg (0.6 mg/rat) or saline at 30 and 37 days of age. Since spontaneous LT rarely occurs before age 75 days, rats were sacrificed at age 65 days to specifically study Tg induced LT. Immunization with NTg induced LT in 31% of the NB rats, but not in the BB subline. LTg did not induce LT in either subline.(ABSTRACT TRUNCATED AT 250 WORDS)

Outer Ring Iodothyronine Deiodinases and Thyroid Hormone Economy: Responses to Iodine Deficiency in the Rat Fetus and Neonate

Female rats were fed a low iodine diet (LID) or the same diet supplemented with KI (IOD) and mated. Plasma TSH, T4 and T3 in thyroid, plasma, and tissues, and 5'-deiodinase activities (5'D) were measured in maternal, fetal, and neonatal samples. Plasma T4 was markedly reduced in LID dams, TSH was increased, and T3 was normal. Placental T4 was decreased to 10%, and placental T3 to 50%. In LID fetuses there was a complete depletion of both extrathyroidal and intrathyroidal stores of T4 and T3. The thyroid responded with increased synthesis and secretion of T3 over T4, as assessed from the T3 to T4 ratios. Near birth, brain T4 and T3 concentrations were only 6.7% and 12% of those in IOD fetuses, despite a marked increase in brain 5'D-II and a T4-sparing decrease in liver and lung 5'D-I. Brown adipose tissue 5'D-II increased 7-fold, and brown adipose tissue T4 and T3 concentrations were only decreased by 50%. After birth, the availability of iodine improved somewhat through maternal milk, and the thyroidal and extrathyroidal pools of T4 and T3 increased, although they remained much lower than those in IOD pups. Brain 5'D-II markedly increased in LID pups, and this together with an increase in plasma and brain T4 ensured almost normal brain T3 during the suckling period. The thyroidal secretion of T3 over T4 continued to be increased in LID pups during the suckling period and appeared to be related to their high circulating TSH levels. Both LID fetuses and newborns can respond to iodine deficiency as adults rats, but the fetus is more sensitive to LID because of its dependence on maternal T4. The success of the adaptative mechanisms in protecting the brain from severe T3 deficiency depends on the supply of iodine, the limiting factor for the synthesis of T4.

Inverse Relation between Iodine Intake and Thyroid Blood Flow: Color Doppler Flow Imaging in Euthyroid Humans

High intake of iodine inhibits iodide trapping, iodide organification, and hormone release from the human thyroid. We investigated whether iodine intake also affects thyroid blood flow, as was suggested by a recent study in euthyroid rats. With a Color Doppler device we made 14 consecutive Duplex-Doppler registrations of both superior thyroid arteries in 10 euthyroid volunteers during baseline iodine intake (1 week), iodine restriction (2 weeks), return to baseline (1 week), and iodine excess (1 week; 80 mumol sodium iodide/day). Vessel diameters and mean flow velocity were measured on videotape recordings by a "blinded" observer. Baseline iodide excretion was 0.88 +/- 0.38 (+/- SD) mumol/day. Mean flow velocity was 13.9 +/- 4.1 cm/s, and vessel diameter was 1.07 +/- 0.22 mm. Blood flow was 7.7 +/- 3.8 mL/min.superior thyroid artery. During the low iodine diet, excretion dropped to 0.49 +/- 0.16 mumol/day, and blood flow increased to 11.0 +/- 5.0 mL/min (P less than 0.001), remaining elevated (10.3 +/- 4.4 mL/min) during the second baseline diet. During high iodide intake, blood flow averaged 5.8 +/- 3.4 mL/min (P less than 0.001), and the expected decrease in thyroid hormone levels and increase in TSH were seen. We conclude that thyroid blood flow responds inversely, and independently from TSH, to changes in iodine intake in euthyroid humans.

Peroxidase-catalyzed bromination of tyrosine, thyroglobulin, and bovine serum albumin: Comparison of thyroid peroxidase and lactoperoxidase

A recent paper (Buchberger, W., 1988, J. Chromatogr. 432, 57) on lactoperoxidase-catalyzed bromination of tyrosine and thyroglobulin stated, without evidence, that thyroid peroxidase (TPO) is able to use bromide as a substrate. This was in disagreement with unpublished experiments previously performed in this laboratory, and we undertook, therefore, to examine this subject further. Highly purified porcine TPO was compared with lactoperoxidase (LPO) and chloroperoxidase (CPO) for ability to catalyze bromination of tyrosine, thyroglobulin, and bovine serum albumin (BSA). The incubation mixture contained 50–100 n m peroxidase, 10–500 μ m 82Br −, tyrosine (150 μ m), thyroglobulin (0.3 or 1 μ m), or BSA (7.5 μ m), and a source of H 2O 2. The latter was either generated by glucose (1 mg/ml)-glucose oxidase (0.5 or 1 μg/ml), or added initially as a bolus (100 μ m). With TPO, formation of organically bound 82Br was undetectable under all conditions in the pH range 5.4–7.0. Lactoperoxidase and CPO, on the other hand, displayed considerable brominating activity. Lactoperoxidase was much more active at pH 5.4 than at pH 7.0 and was more active with BSA as acceptor than with tyrosine or thyroglobulin. The distribution of 82Br among the various amino acids in LPO-brominated thyroglobulin and BSA was determined by HPLC. As expected, monobromotyrosine and dibromotyrosine together comprised the greatest part of the bound 82Br. However, a surprisingly high percentage (20–25%) was present as monobromohistidine. Evidence was also obtained for the presence of a small percentage of the bound 82Br as tetrabromothyronine. Peroxidasecatalyzed bromination probably depends on the oxidation of Br to Br + by the Compound I form of the enzyme. Since oxidation of Br − to Br + requires a stronger oxidant than oxidation of I to I +, our results suggest that Compound I of LPO and of CPO has a higher oxidation potential than Compound I of TPO. In vivo experiments with rats on a low iodine diet injected with 82Br − showed that even under conditions of high stimulation by thyrotropic hormone, there is negligible formation of organic bromine in the thyroid. Measurements of thyroid:serum concentration ratios for 82Br − in similar rats provided no evidence that Br − is a substrate for the iodide transport System of the thyroid.

Abundance of the α-subunits of Gi1, Gi2 and Go in synaptosomal membranes from several regions of the rat brain is increased in hypothyroidism

1. Rats (4 weeks old) were made hypothyroid by treatment with propylthiouracil together with a low-iodine diet for a further period of 4 weeks. Synaptosomal membranes were obtained from six anatomical regions of the brain. 2. The abundances in these membranes of the G-protein alpha-subunits Gi1 alpha, Gi2 alpha and Go alpha were measured by quantitative immunoblotting. 3. Hypothyroidism significantly increased the abundances of all three G-protein subunits in membranes from the cerebral cortex and the striatum. In the medulla oblongata and the hippocampus the abundances of Gi2 alpha and Go alpha were increased significantly. By contrast, in the cerebellum only Go alpha was increased, and in the hypothalamus only Gi2 alpha was increased. 4. It is suggested that this up-regulation of G-protein abundances may modify signalling pathways and may contribute to the functional changes that are observed in the central nervous system in hypothyroidism.

Effect of iodine deficiency and cold exposure on thyroxine 5'-deiodinase activity in various rat tissues

We measured thyroxine 5'-deiodinase I (T(4)5'D-I) activity in thyroid, liver, and kidney and thyroxine 5'-deiodinase II (T(4)5'D-II) activity in brown adipose tissue (BAT) in rats on a low-iodine diet (LID) to test the possibility that increased deiodinase activity in these tissues might contribute to the maintenance of ther serum 3,5,3'-triiodothyronine (T3) level. Control rats received LID plus KI. Experiments were also performed with LID and LID plus KI rats exposed to cold. T(4)5'D-I activity was greatly increased in the thyroids of LID rats but not in liver or kidney. We consider it likely that increased thyroxine (T4)-to-T3 conversion in the greatly enlarged thyroids of LID rats contributed to the maintenance of serum T3. T(4)5'D-II activity in BAT was markedly increased in LID rats and was further greatly increased on cold exposure. However, we were unable to demonstrate an increase in uncoupling protein mRNA levels in BAT in response to cold in LID rats. We attribute this to the very low serum T4 level, which limits substrate availability. This factor also makes it unlikely that BAT contributes to maintenance of serum T3 in LID rats.

Influence of Hypothyroidism Duration on Developmental Changes in the Hypothalamic Factors Implicated in Growth Hormone Secretion in the Male Rat

The effects of hypothyroidism duration on several factors implicated in GH secretion control were studied in the male rat at different maturity stages, ranging from the peripuberal period to adulthood. Thyroid ablation was performed on 22-day-old Wistar male rats maintained on a low iodine diet (T group). Age-paired controls (C group) were fed with the same diet, supplemented with potassium iodide. Subgroups of T and C animals (aged 32, 42, 52, 82 and 112 days) were studied 10, 20, 30, 60 and 90 days after surgery. After pentobarbital anesthesia, jugular blood was withdrawn before and 5 min after an intravenous TRH stimulus, for GH assay. Hypothalamic and pituitary tissues were obtained in order to measure GH, immunoreactive somatostatin (IR-SRIF) and growth hormone-releasing factor (IR-GRF). Growth rate and serum testosterone confirmed that C rats reached sexual maturity by day 30 of the study. Mean +/- SE serum GH (ng/ml) increased (p less than 0.05) in C animals from day 10 (38.5 +/- 5) to day 30 (67.4 +/- 7.3), with no significant variations thereafter. The same time sequence pattern was observed in pituitary GH concentrations. In T rats, both serum and pituitary GH decreased progressively from day 10 to 90, being significantly lower than in C at all times of the study. No GH response to TRH could be found in C groups. In contrast, GH increased significantly (p less than 0.05) in T animals after TRH at days 20 and 30.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of FRTL-5 Cell Growthin Vitrowith That of Xenotransplanted Cells and the Thyroid of the Recipient Mouse*

The present work was designed to compare in vitro cell growth kinetics with in vivo growth under conditions as similar as possible using labeling with [3H]thymidine. To this purpose, FRTL-5 cells were cultured as monolayers and as three-dimensional spheroids embedded in collagen gels and transplanted simultaneously into nude mice treated with perchlorate and a low iodine diet. The growth of the transplants was compared to that of the thyroids in host mice. In the intact thyroid, the fraction of [3H]thymidine-labeled follicular cells (FLC; 24-h labeling) increased sluggishly to a maximum of 10% after 3 weeks of goitrogen exposure, with a subsequent autoregulatory decrease to 3% at 7 weeks. A 4-fold higher FLC was found in six adenomas, indicating focal failure of growth-restraining mechanisms. In nonconfluent monolayer cultures the FLC was as high as 90%, even within large individual clusters where cells are in tight mutual contact. Solid, highly cellular grafts growing from transplanted monodispersed cells showed an average FLC of 20%, which is 5 times higher than the FLC in the identically stimulated mouse thyroid. In collagen-embedded cells, forming three-dimensional spheroids, the mean FLC decreased from 70% at 1 week in vitro (40% in vivo) to 20% at 3 weeks both in vitro and in vivo, suggesting effective auto-regulation of excessive growth in both conditions. However, these FLC were again much higher than the 3% FLC in simultaneously assessed host thyroids. The difference remained throughout the 45-day period studied. We conclude that FRTL-5 cells growing as monolayers and as three-dimensional spheroids in vitro or after xenotransplantation in vivo invariably show much higher proliferation rates under comparable environmental conditions than the normal follicular epithelium in the thyroids of host mice. The one exception is the confluent monolayer with near-zero growth, while densely packed three-dimensional transplants still grow intensively. Although growth-retarding cell to cell interactions are also clearly operative in growing FRTL-5 cells, they are less effective than those dampening the replication rate of the thyrocytes within the monolayer hull of normal follicles. A local failure of these mechanisms, allowing growth rates comparable to those of grafted FRTL-5 cells results in adenoma formation in normal thyroids. These observations call for caution in the transfer of in vitro growth studies with FRTL-5 cells to in vivo conditions prevailing in the normal thyroid.

Restoration of Thyroid Function after Total Thyroidectomy and Quantitative Thyroid Cell Transplantation*

Quantitative thyroid cell transplantation was used to evaluate the reestablishment of thyroid function in surgically thyroidectomized rats. Donor thyroid glands were enzymatically dispersed, and the cells were quantified, serially diluted, and inoculated into the sc fat pads of syngeneic recipient rats 1 day after surgical thyroidectomy. Blood samples were obtained weekly, and serum T3, T4, and TSH levels were determined by RIA. Recipient rats displayed different patterns of restoration of thyroid function depending on 1) the number of thyroid cells inoculated and 2) their dietary iodine intake. We observed cell dose-dependent differences in both the rapidity and extent of thyroid function after cell transplantation. Recipients fed a normal diet and grafted with 6 x 10(4) or more cells reached near-normal levels of all three hormones. In recipients fed a low iodine diet, T4 and TSH levels remained at significantly hypothyroid levels for the duration of the 7-week study. At the highest cell dose examined (3 x 10(5)), however, serum TSH levels began to decline by the sixth to seventh week. In spite of the iodine deficiency, recovery of T3 levels in these recipients paralleled that in recipients fed normal diets. Intact control animals exhibited the anticipated pattern of decreased T4, increased TSH, and essentially unchanged T3. 125I uptake by the grafts was also dependent on the initial number of thyroid cells grafted independent of diet.

Changes in127I Mice Thyroid Follicle Studied by Analytical Ion Microscopy: A Key for the Comprehension of Amiodarone-Induced Thyroid Diseases*

Analytical ion microscopy was used to evaluate the quantitative changes in thyroid 127I mapping induced by amiodarone in relation to the iodine intake. Mice were maintained on a normal diet (NID), on a low iodine diet (LID) or on a high iodine diet (HID) during 3 months. They received daily ip injections of amiodarone or NaI during the last 10 days. Microanalysis of iodine was performed on thyroid sections. In NID mice, the cellular 127I concentration (mean +/- SE: 0.90 +/- 0.06 micrograms/mg) was lower than that of follicular lumina (6.99 +/- 0.17). Amiodarone increased cellular concentration (2-fold) and decreased follicular concentration; NaI induced the same results. In LID mice, both concentrations, which were decreased 60- to 70-fold in comparison with NID, were restored by amiodarone, while NaI enhanced follicular concentration more than amiodarone. In HID mice, the follicular concentration (1.5-fold higher than normal) returned to a normal range with amiodarone, but cellular concentration (2.5-fold lower than normal) remained low, while NaI increased the cellular concentration 3-fold. These data show that iodine released by amiodarone has a bioavailability different from that of NaI. They indicate that thyroid response to amiodarone is related to dietary iodine intake, which, in turn, determines autoregulatory mechanisms. They also raise the question of the possible existence of an intrinsic property of amiodarone which would explain the specificity of its activity on thyroid iodine organification and the development of amiodarone-induced human thyroid diseases.

Histologic changes in tissue components of the hyperplastic thyroid gland during its involution in the rat

Male Fischer rats were fed a low-iodine diet containing thiouracil for 21 days to produce hyperplastic thyroid glands, and then fed a high-iodine diet for various time intervals, from 5 hr to 180 days, in order to study the morphological changes that occur during involution. Thyroids were fixed by perfusion fixation and embedded in Epon. Sections were examined by light microscopy. Initially at 0 days of involution (at the time of the change to the high-iodine diet), follicular lumens were very narrow and capillary lumens were very wide. The capsule was thick and infiltrated with mononuclear leukocytes. No obvious changes occurred for 1 day after the change in diet, but shortly thereafter capillary lumens began to narrow. By 4 days, most capillary lumens were close to normal size; capillaries formed a more or less normal bed except that many were embedded in a relatively thick or wide interfollicular matrix. This matrix was largely gone by 21 days. Between 1 and 21 days, follicular lumens dilated progressively as colloid accumulated. The density of staining of the accumulated colloid varied from follicle to follicle, and this variation was also observed in older controls. Inflammatory cells gradually disappeared from the capsule and most were gone by 15 days. Starting at approximately 15 days and continuing to 180 days, one or more disintegrating cells were found in some lumen profiles. Colloid goiters were not observed in these rats even after several months of involution. Some lumens were rather large, however, and small fractions of the follicles, both small and large, were bounded by flat cells and resembled "cold" follicles morphologically.