Thyroid Hormone Deprivation Research Articles

EFFECT OF THYROID HORMONE ON GROWTH: Lessons from the Syndrome of Resistance to Thyroid Hormone

Thyroid hormone deprivation results in deleterious effects on bone growth. The delayed bone development is mediated by a direct effect of thyroid hormone on bone and an indirect effect of the hormone on GH release and IGF-1 action. Both TR alpha and TR beta are expressed in bone cells. To examine the role of TR beta on bone, we have reviewed the growth abnormalities in the human syndrome of RTH caused by mutations in the TR beta gene. The mutant TR beta reduces the tissue responsiveness to thyroid hormone, producing in some tissues variable degrees of thyroid hormone deprivation. With regard to bone, relative thyroid hormone deficiency caused by the mutant TR beta produces short stature and delayed bone growth but does not attenuate growth to the extent that absolute thyroid hormone deficiency does. These observations indicate that an intact TR beta is required for normal bone development and growth.

Consequences of thyroid hormone deficiency induced by the specific ablation of thyroid follicle cells in adult transgenic mice

The herpes simplex type 1 virus thymidine kinase (HSV1-TK) reporter gene was coupled to a bovine thyroglobulin promoter (TG-tk construct). Within the thyroid glands of transgenic mice expression was confined to thyroid follicle cells. Infusion of Ganciclovir (9-[(1,3-dihydroxy-2-propoxy)methyl]guanine) to 8 to 12 week transgenic females led to the complete loss of thyroid HSV1-TK activity (at 3 to 4 days) and thyroid follicles (between 7 and 14 days). During the first 5 days of treatment a single reciprocal oscillation in circulating thyroxine (T4) and TSH levels occurred. By 14 days the circulating tri-iodothyronine (T3) and T4 levels of all treated animals were below the detection limits of the assays, while TSH levels were elevated ten-fold and continued to increase thereafter. During 14 days of treatment the thyroids regressed, protein content fell by 80-90% and the C cells, normally dispersed within the central region of each gland, came together in aggregates. Pituitary GH levels in females rose and fell back to normal within 14 days and between 14 and 28 days fell to a level comparable with that of GH-deficient lit/lit mice. The levels of hepatic GH receptor mRNA and the predominant 6.6 kb T3 receptor mRNA were unaffected by thyrocyte ablation. Thyrocyte ablation had no effect on the level of prolactin (Prl) receptor mRNA in females, but increased Prl receptor mRNA levels in males and eliminated group 1 major urinary protein (MUP).mRNA in females. T4 replacement reversed the effects of thyrocyte ablation on MUP mRNA in females and on Prl receptor mRNA in males.2+ Despite the many physiological changes induced by thyrocyte ablation, ablated mice have been maintained for up to 1 year without thyroid hormone supplementation. T4-deficient females were normally fertile and carried pups to term. Although transgenic males expressed HSV1-TK ectopically in spermatids and spermatozoa at levels similar to thyrocyte levels, a rate of Ganciclovir infusion which successfully ablated the thyrocytes did not affect the testis. As an alternative to infusion by minipump, thyrocyte ablation could be achieved by 6 twice-daily injections of Ganciclovir, at a level of 112 micrograms Ganciclovir/g body weight per day, and fetuses in utero could be thyrocyte ablated by administering 50 or 15 micrograms/g body weight per day to pregnant females between days 14 and 18 of gestation. These data demonstrate the potential value of transgenic thyrocyte ablation in the study of the effects of thyroid hormone deprivation.

Effects of thyroid hormone deprivation on immunity in postmetamorphic frogs

Previously, we showed that sodium perchlorate treatment of larval frogs ( Xenopus laevis) interferes with the normal expansion of T and B lymphocytes and development of an adult-type T-cell population. It was unclear whether these effects resulted from preventing metamorphosis or from long-term thyroid hormone (TH) deprivation. To try to distinguish between these possibilities, we have now studied the effects of perchlorate treatment beginning immediately after metamorphosis. After 5 months, treated animals, but not untreated controls, had large thyroid goiters, were significantly smaller, and had significantly fewer erythrocytes, thymocytes, and splenocytes. Although the number of IgM − splenocytes and thymocytes was reduced, the estimated percent of major histocompatibility complex (MHC) class II + cells within this subset was not significantly different from that of controls. Furthermore, splenocytes from perchlorate-treated frogs could respond normally by [ 3H]TdR incorporation to the T-cell mitogens, phytohemagglutinin-P (PHA) and concanavalin A (Con A). Thus, unlike perchloratetreated larvae, perchlorate-treated juveniles appear to be able to develop T cells with an adult phenotype competent to respond to activation and proliferation signals.

Effects of Neonatal Hypothyroidism on Rat Brain Gene Expression

To define at the molecular biological level the effects of thyroid hormone on brain development we have examined cDNA clones of brain mRNAs and identified several whose expression is altered in hypothyroid animals during the neonatal period. Clones were identified with probes prepared by subtractive or differential hybridization, and those corresponding to mRNAs altered in hypothyroidism were further studied by Northern blot analysis. Using RNA prepared from whole brains, no effect of hypothyroidism was found on the expression of the astroglial gene coding for glial fibrillary acidic protein. Among genes of neuronal expression, no significant alterations were found in the steady state levels of mRNAs coding for neuron-specific enolase, microtubule-associated protein-2, Tau, or nerve growth factor. N-CAM mRNA increased slightly in hypothyroid brains. In contrast a 2- to 3-fold decrease was found in the mRNA coding for a novel neuronal gene, RC3. This is the first neuronal gene known to be significantly altered at the mRNA level by thyroid hormone deprivation. The abundance of the mRNAs for the major myelin proteins proteolipid protein, myelin basic protein, and myelin-associated glycoprotein, expressed by oligodendrocytes, were also decreased in hypothyroid brains. Developmental studies on RC3 and myelin-associated glycoprotein expression indicated that the corresponding mRNAs accumulate in the brain of normal rats during the first 15-20 days of neonatal life. A similar accumulation occurred in hypothyroid brains, but at much reduced levels. The results demonstrate that thyroid hormone controls the steady state levels of particular mRNAs during brain development.

Selective persistent reduction in choline acetyltransferase activity in basal forebrain of the rat after thyroid deficiency during early life

The effect of thyroid deficiency on the activity of choline acetyltransferase (ChAT; the marker for cholinergic neurons) was studied in different parts of the rat brain at ages 5, 10, 15 and 25 days, and at day 130 following 102 days of rehabilitation. During normal development, the activity of ChAT increased in the cerebral cortex, hippocampus and basal forebrain, and decreased in the cerebellum. Neonatal thyroid deficiency resulted in a marked retardation of the developmental patterns of the enzyme activity. In the hippocampus the effect diminished with age even during the period of thyroid hormone deprivation, while in the cerebral cortex and cerebellum the enzyme activity was restored to normal only after rehabilitation. In contrast, ChAT activity in the basal forebrain remained persistently reduced in comparison with controls. The results indicate that neonatal thyroid deficiency causes selective irreversible biological damage to subcortical cholinergic neurons.

Characteristics of thyroxine 5'-deiodination in cultured human placental cells. Regulation by iodothyronines.

Human and rat placental homogenates convert L-thyroxine (T4) to 3,5,3'-L-triiodothyronine (T3) via a pathway termed type II iodothyronine deiodination. To study regulation of this pathway, cell dispersions were prepared from human placental chorionic-decidual membrane. Dispersed cells deiodinated T4 and 3,3',5'-triiodothyronine (rT3), but not T3, at the 5' position. The reaction was only slightly inhibited by 1 mM 6-n-propylthiouracil, enhanced by dithiothreitol, and substantially inhibited by 50 nM iopanoic acid. Incubation of the cells in thyroid hormone-depleted medium induced a near doubling of T4 5'-deiodination in 36-48 h, with a significant rise seen as early as 12 h. Addition of T4, rT3, or T3 to hormone-depleted medium impaired the rise in type II deiodination in a dose-dependent fashion. T4 and rT3 were equipotent in this regard, and T3 was 2-3 times less potent. T4 was effective in physiological concentrations, 6.5-13 nM in medium containing 10% calf serum, and the effect of T4 was not due to its conversion to either T3 or rT3. In cells with deiodinase activity raised by 48 h incubation in thyroid hormone-depleted medium, addition of T4, T3, or rT3 reversed the increase in 8-24 h. Secretion of prolactin and beta hCG by the dispersed cells was not substantially affected by thyroid hormone deprivation. The increase in type II deiodination during thyroid hormone deprivation appears to depend on a signal from the thyroxine molecule, per se, and could potentially defend intracellular, and/or circulating, T3 pools in pathological states of mild-to-moderate hypothyroxinemia.

Growth hormone response to thyrotropin-releasing hormone in the urethane-anesthetized rat: effect of thyroid status.

To evaluate the role of thyroid hormones in regulation of the GH-stimulatory effects of TRH and human pancreatic GH-releasing factor (hpGRF-40), we studied the plasma GH responses to these secretagogues under conditions of thyroid hormone deprivation and replacement in the urethane-anesthetized rat. In euthyroid control rats, TRH (1 microgram/kg) elicited a small transient rise in plasma GH, which peaked at 2-5 min and returned to basal by 20 min. In chronically hypothyroid rats (10 weeks after thyroidectomy), intrapituitary GH was markedly depleted to less than 0.1% of normal, and TRH was completely ineffective in eliciting a plasma GH response to TRH. In chronically hypothyroid rats given T4 (20 micrograms/kg daily) for 4 days, intrapituitary GH was partially repleted, and the GH response to TRH was markedly enhanced compared to that in euthyroid rats. The extent to which the GH response to TRH was enhanced by chronic hypothyroidism, followed by short term T4 treatment, depended on the duration of T4 administration. The plasma GH response was greatest after 1-3 days of T4 treatment; treatment for 7 days, on the other hand, suppressed the GH response to below that of euthyroid rats. The minimal duration of hypothyroidism which, in combination with short term (2 days) T4 treatment, enhanced the plasma GH response to TRH, was 6 weeks, with 8 weeks or longer being optimal. The effect of TRH on plasma GH was dose dependent in thyroidectomized rats given T4 for 2 days; the lowest maximally stimulatory dose was 10 times less than that in the euthyroid rat (1 vs. 10 micrograms/kg). The GH-stimulatory effect of TRH in thyroidectomized rats given T4 for 2 days was abolished by the simultaneous administration of SRIF (40 micrograms/kg). That the failure of TRH to stimulate GH release in the chronically hypothyroid rat may have been the consequence of a depletion of intrapituitary GH available for release was suggested by the finding that in parallel studies, hpGRF-40, a more potent stimulator of GH release in the euthyroid rat, was also without effect. In contrast to the GH response to TRH, the GH response to hpGRF-40 was only partially restored by T4 treatment of chronically hypothyroid rats for 2 days. We conclude that chronic thyroid hormone deficiency selectively sensitizes the somatotroph to TRH. Short term thyroid hormone replacement is needed to replete intrapituitary GH and allow the expression of this enhanced GH secretory response to TRH. More prolonged treatment with T4, on the other hand, appears to desensitize the somatotroph to TRH.

The effects of thyroid hormone deprivation in vivo and in vitro on growth hormone (GH) responses to human pancreatic (tumor) GH-releasing factor (1-40) by dispersed rat anterior pituitary cells.

Anterior pituitary cells from euthyroid and hypothyroid male rats have been cultured as monolayers for 3 days with or without 5 nM T3 and stimulated with either human pancreatic GH-releasing factor 1-40 (hpGRF), TRH, or the Ca2+ channel ionophore A23187. Basal GH secretion was reduced in the hypothyroid cultures (P less than 0.001) and basal TSH secretion increased (P less than 0.001). Culture with T3 increased GH secretion and intracellular GH content in euthyroid and hypothyroid cultures but suppressed TSH secretion with no effect on intracellular TSH content in either euthyroid or hypothyroid cultures. hpGRF released more GH from euthyroid [3.52 +/- 0.2 (SE) micrograms/6 h X 10(5) cells] than hypothyroid cultures of (0.17 +/- 0.01 micrograms/6 h X 10(5) cells, P less than 0.001) without a change in ED50 (approximately 0.02 nM). The reduction in hpGRF-induced GH release remained significant when corrected for the reduced intracellular GH content in the hypothyroid cultures. hpGRF-induced GH release also declined relative to A23187-induced GH release in hypothyroid cultures. Culture with 5 nM T3 doubled maximum hpGRF-induced GH release in euthyroid cultures and increased maximum release 10-fold in hypothyroid cultures without altering the ED50 of hpGRF action. In contrast, T3 suppressed TRH-induced TSH release in euthyroid cultures but was without effect on TRH-induced TSH release in the hypothyroid cultures. T3 did not effect the ED50 of TRH action (2-5 nM). In summary, hypothyroid rat anterior pituitary cells in culture have a reduced maximal GH response to hpGRF, but the same ED50. hpGRF activity can be partially restored by physiological concentrations of T3 in vitro.

Control of glycogen metabolism in the developing fetal lung.

Summary: The control of glycogen metabolism was studied in the lung of the developing rat or rabbit fetus by means of hormonal suppression (in utero decapitation, or thyroidectomy, or drug administration) and hormone administration to intact or hormone-deprived fetuses. In the rat fetus, in utero decapitation plus maternal adrenalectomy did not impair lung glycogen accumulation between days 17.5 and 19.5. On the contrary, glycogen breakdown, which occurs after day 20.5 in the rat and day 27 in the rabbit, was impaired, at least partly, by decapitation of the 17.5-day-old rat fetus and of the 23-day-old rabbit fetus. On the average, on day 21.5 the glycogen content of the lungs of the decapitated rat fetus was 323 ± 33 mg/g proteins versus 195 ± 14 mg/g proteins in the lungs of their controls. Thyroid hormone deprivation did not reproduce the effect of decapitation. Phosphorylase activity, which increases near term, still increased after decapitation; however, on the average, it was reduced by approximately 25%. Cortisol injection to decapitated fetuses restored normal glycogen breakdown without restoring phosphorylase activity. Growth hormone administered to decapitated fetuses restored a normal phosphorylase activity, but the lung glycogen content remained higher than in controls.

Effects of thyroidectomy and thyroxine on plasma growth hormone and insulin levels in rats.

Plasma growth hormone and insulin levels were measured in normal, thyroidectomized (TX), and TX rats treated with 1-thyroxine (1-T4) or rat-growth hormone (r-GH). Although normal growth was observed 5 days after surgery, a decrease of circulating levels of GH was evidenced as early as 5 days after the operation. However, the plasma insulin levels were not affected by the 5th, 10th and 15th day of thyroidectomy. With a more prolonged thyroid hormone deprivation, the plasma insulin levels of TX rats remained lower than those of age-paired controls. Treatment of hypothyroid rats with 0.05 micron 1-T4 for 10 days induced growth and an increase in circulating GH levels, but had no effect on the low levels of plasma insulin of TX rats. Treatment of TX animals with 0.10 or 0.20 micron 1-T4, or 100 micron r-GH during 10 days caused plasma insulin levels to shift toward the values of their weight-paired controls. However, treatment of hypophysectomized (HX) rats with 0.20 micron T4 during 8 days did not change the circulating levels of plasma insulin. It is suggested that thyroid hormone deprivation results in a decrease of circulating levels of insulin secondary to a deficiency in growth hormone secretion. In addition, normal GH secretion appears to be required for normal pancreatic insulin secretion to occur.

Rapid effects of single small doses of L-thyroxine and triiodo-L-thyronine on growth hormone, as studied in the rat by radioimmunoassy.

The effects of thyroid hormone deprivation and restitution on growth hormone (GH) economy have been studied in the rat by means of a specific radioimmunoassay. The pituitary GH content and the plasma GH levels before and during stimulation with pentobarbital ("PB-test") were studied in male rats at different intervals after surgical thyroidectomy (T), and in T rats at different time intervals after the ip injection of 0.20, 1.75, and 5.0 mug thyroxine (T4) or 0.05, 0.10, 0.20 and 1.0 mug triiodothyronine (T3), all doses being referred to 100 g body wt. Pituitary GH content decreased very rapidly after T, a difference being shown at the end of the shortest time interval studied (24 h); 24 days after T, pituitary GH content was 0.3 percent or less of the pre-T level, the basal plasma GH was lower than in intact controls and an increase in plasma GH during PB-stimulation was no longer observed. When rats T for 30 days or longer were injected once with T4 or T3, pituitary GH content increased; basal plasma GH levels increased also and a positive response to PB was observed. An effect on pituitary GH content could be observed as soon as 2 h after the ip injection of 1.0 mug T3, or 6 h after 5.0 mug T4. The "latent period" was somewhat longer when lower doses of the hormones were used. Effects of a single 0.10 mug T3 dose could be detected within 12 h L-T3 appeared to be at least 9 times more potent in vivo tha T4, as assessed from the effect on pituitary GH. The mea-urement by RIA of changes in GH content of the rat pituitary may thus provide the most adequate parameter available at present (other than suppression of TRH-induced TSH release) for a biological effect in vivo of single doses of the thyroid hormones, a measurement clearly related to an important physiological role.