Prenatal exposure of the fetal rat to excessive L-thyroxine or 3,5-dimethyl-3'-isopropyl-thyronine produces persistent changes in the thyroid control system

Studies were conducted to determine if brief exposure, in utero, to high levels of T4 or to the synthetic thyromimetic agent 3,5-dimethyl-3'-isopropyl-L-thyronine (DIMIT) can produce permanent disruption of the thyroid control system in a manner analogous to the changes in the "set point" reported to occur due to neonatal T4 exposure in the "neo-T4 syndrome". If such a change were to occur, it could explain the persistent thyroid disturbances seen in the progeny of hypothyroid mother rats. These latter progeny are exposed in utero to both low and high serum T4 levels. Maternal T4 treatment produced a 4-fold elevation in fetal serum T4 accompanied by a large decrease in serum TSH levels. The brief treatment in utero with high doses of T4 or of DIMIT resulted in higher neonatal mortality and the T4-treatment produce subsequent growth stunting. These treatments resulted in suppression of the fetal/neonatal thyroid which was very apparent at 5 days of age. At 30 days post-partum, the thyroid control system of the progeny of the T4 and DIMIT-treated animals was still abnormal with low serum T4 levels accompanied with normal serum TSH and T3 levels. At 60 days of age, serum T4 levels remained low in the progeny of the T4-treated animals and the TSH response to TRH was subnormal in both the progeny of the T4-treated and the DIMIT-treated animals. However, serum and pituitary TSH and serum T3 were normal. The thyroid control system of the rat is sensitive to prenatal exposure to hyperthyroxinemia as it is to postnatal exposure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Estrogen effects on thyroid iodide uptake and thyroperoxidase activity in normal and ovariectomized rats

Sex steroids interfere with the pituitary-thyroid axis function, although the reports have been controversial and no conclusive data is available. Some previous reports indicate that estradiol might also regulate thyroid function through a direct action on the thyrocytes. In this report, we examined the effects of low and high doses of estradiol administered to control and ovariectomized adult female rats and to pre-pubertal females. We demonstrate that estradiol administration to both intact adult and pre-pubertal females causes a significant increase in the relative thyroid weight. Serum T3 is significantly decreased in ovariectomized rats, and is normalized by estrogen replacement. Neither doses of estrogen produced a significant change in serum TSH and total T4 in ovariectomized, adult intact and pre-pubertal rats. The highest, supraphysiological, estradiol dose produced a significant increase in thyroid iodide uptake in ovariectomized and in pre-pubertal rats, but not in control adult females. Thyroperoxidase activity was significantly higher in intact adult rats treated with both estradiol doses and in ovariectomized rats treated with the highest estradiol dose. Since serum TSH levels were not significantly changed, we suggest a direct action of estradiol on the thyroid gland, which depends on the age and on the previous gonad status of the animal.     

Thyroid hormone regulation of messenger ribonucleic acid encoding thyrotropin (TSH)-releasing hormone is independent of the pituitary gland and TSH

Cellular levels of mRNA encoding pro TRH in the rostral paraventricular nucleus are reduced by thyroid hormones. To determine whether this regulatory effect of thyroid hormones requires a functional pituitary gland or, specifically, TSH, we examined the effect of T3 on proTRH mRNA in hypophysectomized, thyro-parathyroidectomized male rats with or without bovine TSH replacement. Hypophysectomy plus thyro-parathyroidectomy reduced serum T4 and TSH to undetectable levels in all animals and elevated TRH mRNA in the paraventricular nucleus over that of sham-operated animals. Eleven consecutive daily injections of T3 significantly reduced TRH mRNA levels in both sham controls and thyro-parathyroidectomized rats. However, 11 daily injections of bovine TSH (1 U/day) failed to alter the effect of T3 on TRH mRNA levels. These results demonstrate that the regulatory influence of thyroid hormones on the biosynthesis of TRH within the thyrotropic center of the brain is independent of the pituitary gland and of TSH.     

Disturbance of thyroidal iodine metabolism in BB/W rat.

To investigate the thyroid function in Bio-Breeding Worcester (BB/W) rats, we have examined the iodine metabolism, serum TSH and thyroid hormone levels in 8- and 16-week-old BB/W and normal Wistar (W) rats. At 8 weeks of age, serum TSH levels were significantly higher in BB/W rats than in W rats, although there was no difference in the serum levels of free T3 and free T4. Furthermore, the thyroidal radioactive iodine incorporation at 48 h was significantly lower in BB/W rats, suggesting that they might have some defects in iodine organification. At 16 weeks of age, serum TSH levels were also significantly higher in BB/W rats than in W rats. Furthermore, serum TSH levels in 16-week-old BB/W rats were significantly higher than in 8-week-old BB/W rats. The thyroid weight was significantly greater in BB/W rats, probably due to the increased serum TSH. The thyroidal radioactive iodine uptake at 48 h and the iodine content in the thyroid homogenates were significantly lower in BB/W rats. These results suggest that BB/W rats have some defect in iodine metabolism resulting in impaired thyroid hormone synthesis.     

Goitres in rats fed polychlorinated biphenyls.

Rats fed a polychlorinated biphenyl (PCB) mixture in a high- or low-iodine diet (HID or LID respectively) for 15 days had thyroid enlargement, low serum thyroxine (T4), and high serum thyrotropin concentrations. Although binding of thyroid hormones to serum proteins was reduced in PCB-fed animals, the free T4 index (reflecting free T4 in serum) was less in these rats. Both serum triiodothyronine (T3) and the free T3 index were elevated in rats fed PCB in HID. LID-maintained rats elevated serum T3 concentrations but the free T3 index was similar to that in HID-fed rats, owing to enhanced binding of thyroid hormone to serum proteins. Addition of PCB to LID reduced serum T3 levels but did not alter the free T3 index because binding was less. In rats fed HID containing PCB, thyroid 131I uptake was increased.     

Lowering of T3 and rise in reverse T3 induced by hyperglucagonemia: altered thyroid hormone metabolism, not altered release of thyroid hormones

Recently we reported that hyperglucagonemia induced by glucagon infusion causes a decline in serum T3 and a rise in reverse T3 in euthyroid healthy volunteers. These changes in T3 and rT3 levels were attributed to altered T4 metabolism in peripheral tissues. However, the contribution of altered release of thyroid hormones by the thyroid gland could not be excluded. Since the release of thyroid hormones is inhibited in primary hypothyroidism and is almost totally suppressed following L-thyroxine replacement therapy, we studied thyroid hormone levels for up to 6 hours after intravenous administration of glucagon in subjects with primary hypothyroidism who were rendered euthyroid by appropriate L-thyroxine replacement therapy for several years. A control study was conducted using normal saline infusion. Plasma glucose rose promptly following glucagon administration demonstrating its physiologic effect. Serum T4, Free T4, and T3 resin uptake were not altered during both studies. Glucagon infusion induced a significant decline in serum T3 (P less than 0.05) and a marked rise in rT3 (P less than 0.05) whereas saline administration caused no alterations in T3 or rT3 levels. Thus the changes in T3 and rT3 were significantly different during glucagon study when compared to saline infusion. (P less than 0.01 for both comparisons). Since, the release of thyroid hormones is suppressed by exogenous LT4 administration in these subjects; we conclude that changes in serum T3 and rT3 observed following glucagon administration reflect altered thyroid hormone metabolism in peripheral tissues and not altered release by the thyroid gland.     

Activity of the hypothalamo-pituitary ovarian axis in hypothyroid rats with or without triiodothyronine replacement

The hypothalamic pituitary ovarian axis in adult female rats with 131-I induced hypothyroidism was studied before and after triiodothyronine (T3) replacement. Forty days after 131-I, hypothyroid (H) rats showed irregular cycles with predominantly diestrous vaginal smears, atrophied and underweight ovaries, and decreased serum T3, T4, LH and estradiol (E2). T3 replacement restored normal cycles and ovary weight and increased serum E2 levels above control values, while LH levels remained below the limit of detection of the RIA. The GnRH stimulation test performed on the day that the rats exhibited diestrous vaginal smears elicited a greater increase in FSH than in LH in H rats and a greater increase in LH than in FSH in both H-T3 treated and control rats. The data suggest that the lack of thyroid hormones in adult female rats seems to produce a reversion of sexual hormones to a prepubertal pattern, while T3 treatment restored normal estrous cycles and ovarian function.     

Inhibition of pituitary-gonadal function in male rats by a potent GnRH antagonist

The inhibitory effects of the potent GnRH antagonist, [Ac-D-pCl-Phe1,2,D-Trp3,D-Arg6,DAla10]GnRH (GnRHant) upon pituitary-gonadal function were investigated in normal and castrated male rats. The antagonist was given a single subcutaneous (s.c.) injections of 1-500 micrograms to 40-60 day old rats which were killed from 1 to 7 days later for assay of pituitary GnRH receptors, gonadal receptors for LH, FSH, and PRL, and plasma gonadotropins, PRL, and testosterone (T). In intact rats treated with low doses of the antagonist (1, 5 or 10 micrograms), available pituitary GnRH receptors were reduced to 40, 30 and 15% of the control values, respectively, with no change in serum gonadotropin, PRL, and T levels. Higher antagonist doses (50, 100 or 500 micrograms) caused more marked decreases in free GnRH receptors, to 8, 4 and 1% of the control values, which were accompanied by dose-related reductions in serum LH and T concentrations. After the highest dose of GnRHant (500 micrograms), serum LH and T levels were completely suppressed at 24 h, and serum levels of the GnRH antagonist were detectable for up to 3 days by radioimmunoassay. The 500 micrograms dose of GnRHant also reduced testicular LH and PRL receptors by 30 and 50% respectively, at 24 h; by 72 h, PRL receptors and LH receptors were still slightly below control values. In castrate rats, treatment with GnRHant reduced pituitary GnRH receptors by 90% and suppressed serum LH and FSH to hypophysectomized levels. Such responses in castrate animals were observed following injection of relatively low doses of GnRHant (100 micrograms), after which the antagonist was detectable in serum for up to 24 h. These data suggest that extensive or complete occupancy of the pituitary receptor population by a GnRH antagonist is necessary to reduce plasma gonadotropin and testosterone levels in intact rats. In castrate animals, partial occupancy of the available GnRH receptor sites appears to be sufficient to inhibit the elevated rate of gonadotropin secretion.     

Development of young rats and rabbits exposed to a strong electric field

Body growth and circulating levels of hormones were assessed in young rats and rabbits exposed to a 50-Hz electric field of 50 kV/m. Eight-week-old male rats were exposed 8 h/day for 4 weeks and rabbits were exposed 16 h/day from the last 2 weeks of gestation to 6 weeks after birth. The body and the organ growth of exposed rats were not statistically different from those of sham-exposed controls. No important differences from controls were observed in plasma levels of corticosterone, TSH, ACTH, and T4 or in adrenal levels of epinephrine, norepinephrine, and corticosterone although T3 was slightly, but significantly, decreased. No large histological changes in the thyroid or adrenals were noted. In rabbits, organ and body weights of exposed animals were comparable to those of controls. Plasma levels of various hormones (ACTH, GH, T3, T4, corticosterone, cortisol), serum glucose, triglycerides, and cholesterol were not significantly altered. Adrenal content of cortisol was lower, however, in exposed rabbits. No histological changes of the thyroid or adrenal glands were observed.     

Pituitary-testicular axis abnormalities in immature male hypothyroid rats

The pituitary-testicular disturbances which follow the onset of hypothyroidism were studied in immature male Wistar rats rendered hypothyroid by treatment with methimazole (MMI) given in drinking water, starting at 40 days of age. Half of the animals continued on MMI (MMI group) up to 140 days of age; the remaining rats were withdrawn MMI at 100 days and injected thereafter s.c. with 3 micrograms of T3 daily, during the last 40 days (MMI + T3 group). Ten rats were used as controls (C group). Hypothyroidism induced in immature animals significantly decreased serum T4, T3, LH, PRL, and testosterone levels, and also impaired the normal growth of body and sex accessory glands. T3 replacement therapy helped to normalize serum hormonal levels, but the body and sex accessory gland weights were not fully corrected. Hypothyroidism also reduced the [125I]LH/hCG binding sites of testicular homogenates. T3 replacement was not able to improve the binding; nonetheless, the hormone-receptor affinity constant remained unaltered among the groups. Leydig cell responsiveness to hCG stimulation in vitro (0-82 nM) showed impaired testosterone production in the MMI group (25% of that found in the C group) and also in the MMI + T3 group (80% of that found in the C group). These data demonstrate that induction of hypothyroidism in the immature male rat leads to alterations in serum LH, PRL and testosterone levels, and suggest that thyroid hormones have a modulating action on the testis as far as LH-mediated testosterone secretion is concerned.     

Normal functions of the thyroid gland of the pygmy goat.

Normal values were obtained for blood serum PBI, T3 index. T4, and cholesterol in a colony of pygmy goats (n equal to 55) of mixed sex and age. Serum PBI values averaged 8.1 plus or minus 1.2 mug/dl with no significant sex differences. The mean T3 index and T4 value were 1.1 plus or minus 0.1 and 7.2 plus or minus 1.1 mug/dl, respectively, with no sex differences. The mean serum cholesterol value was 90.0 mg/dl, with sex differences apparent. Serum cholesterol averaged 84.9 mg/dl (n equal to 44) for females and was significantly higher in intact males (97.4 mg/dl; n=8) and significantly lower in castrate males 69.2; n=6. There was a consistent and significant increase in cholesterol values with age in females, an unexplained phenomenon also observed in humans. There was no evidence of thyroid malfunction in the animals studied.     

Thyroid status in the obese syndrome of rats

The thyroid function was explored by comparing serum total and free iodothyronine levels in young male genetically obese Zucker rats and in their lean littermates, aged from 6 to 8 weeks old. Total and free thyroxine (T4) and 3,5,3'triiodothyronine (T3) levels were significantly decreased in obese rat serum while total 3,3',5'-triiodothyronine (rT3) remained constant. Radioactive T4 half life is slower in the plasma of obese rats. Peripheral synthesis of T3 from deiodination of T4 is also decreased in obese rat liver homogenate. These modifications produce changes in liver mitochondria oxidative phosphorylation and in marker enzyme activity, which are usually associated with hypothyroidism and hypothalamic disturbances. Genetic obesity probably involves activation of peripheral deiodination of T4 to rT3 which induces biochemical and metabolic changes.     

Iodoamino acid distribution and weight of the thyroid gland, T4 serum level and T4 metabolism in relation to diet, body fat content and age of rats

Rats were rendered obese by feeding them a fatty diet (HFD, fat: 50% of weight). At the end of the experimental period the animals were divided, also the control rats, which were fed a low-fat diet (LFD, fat: 3% of weight), in light and heavy weight groups. The heavy and obese HFD groups showed, unlike the light LFD-animals, equal absolute but significantly lower relative thyroidal weights. The absolute thyroidal weights of light and heavy animal groups, which received in each case the same diet, were identical, the relative thyroidal weights of the heavy rats on the other hand decreased significantly. The iodoamino acid distribution in the thyroid of rats showed no variation in animals fed various diets and differed in body mass and fat content. The T4 serum levels of the HFD-, in comparison to the LFD-animals increased significantly. Between light and heavy animals in equal diet groups no differences were obtained for the T4 serum values. The serum T3 levels of LFD- and HFD-rats were also equal. The heavy HFD- showed in comparison to the light LFD-animals a significantly lower T4 clearance and in the higher age groups a significantly extended T4 half-life time. The HFD- and LFD-rats with approximately equal body mass and body fat content showed no differences in T4 half-life time and T4 clearance rate depending on diet. A relation between higher body fat content and increased serum T4 levels as a possible adaption to obesity in heavy HFD-rats is discussed. By the comparison of younger and older rats in the most investigated diet and weight groups the older animals showed a decreasing tendency for K and TD/100 g KM and a significant decrease in the clearance rate and in the TD for T4, related to body mass. An influence of diet, body mass or fat content on the decrease of the T4 metabolism of rats with increased age is not evident. The T4 serum levels were not different in dependence on age, but the free T4 serum level was significantly lower in the older rats.     

Hyperresponsiveness of residual thyroid lobe of hemithyroidectomised rat to thyrotropin

The effects of hemithyroidectomy and thyrotropin administration on rat thyroid gland function were studied in adult male rats. Immediately after surgery or sham operation rats were treated daily with 0.12 IU of bovine thyrotropin (TSH) for 3 or 5 days. In control rats TSH dose applied resulted in an increase in serum T4 level at day 5 of experiment. Serum thyroxine concentration markedly decreased in sham operated and hemithyroidectomised rats, an effect observed at days 3 and 5 of experiment. TSH administration had no effect on serum T4 concentration in sham operated rats while in hemithyroidectomised animals such a treatment resulted in a marked increase in serum T4 level, a phenomenon observed in both time intervals studied. The reasons for hemithyroidectomy-induced hyperresponsiveness of rat thyroid residual lobe to thyrotropin are unknown.     

Amiodarone inhibits T4 to T3 conversion and alpha-glycerophosphate dehydrogenase and malic enzyme levels in rat liver

Amiodarone has been found to decrease serum T3 by blocking peripheral T4 5'-deiodinase. This reduction in T3 levels may contribute to the effectiveness of this drug in moderating cardiac arrhythmias. To further characterize the effect of amiodarone on thyroid hormone metabolism and biological action, male Sprague-Dawley rats were thyroidectomized and then fed 500 ug T4 or 50 ug T3 and 500 mg amiodarone/kg of powdered diet for 6 to 8 weeks. Hepatic and cardiac levels of T4, T3, alpha-glycerophosphate dehydrogenase (GPD) and malic enzyme (ME) were used as indicators of thyroid hormone availability and action at the cellular level. Conversion of T4 to T3 was measured in liver homogenates. Serum TSH, T4 and T3 were also measured. Amiodarone reduced hepatic GPD and ME in thyroidectomized rats receiving dietary T4. Liver T4 levels were significantly increased by amiodarone and the T3/T4 ratio was reduced (P less than .05). Amiodarone inhibited hepatic T4 to T3 conversion and decreased serum T3. The decreased T3 action at the cellular level, indicated by the reduction in hepatic GPD and ME, is not due to pharmacologic effects of amiodarone since these enzyme levels were not affected by amiodarone in thyroidectomized rats replaced with T3.     

Thyroid hormone levels in rats exposed to alcohol during development

Maternal ingestion of alcohol appears to cause a pattern of congenital anomalies with a reduction of pre- and postnatal growth in the offspring. In order to study the possible implication of thyroid function in the effects of pre- and/or postnatal exposure to alcohol, we have studied serum thyroxine (T4) and triiodothyronine (T3) levels in rats from alcohol-fed mothers during the postnatal period (0-50 days). Blood alcohol levels of ethanol-treated pregnant rats were approximately equal to 20-25 mM and their serum T4 levels were decreased, compared with the pair-fed controls, at 15 and 21 days of gestation. No significant changes were observed in T3 levels. Prenatal alcohol exposure was associated with a decrease in both T4 and T3 levels in pups at birth. Although T4 levels continued reduced in the 40-50 days of the postnatal period, no clear effects were observed on T3 levels during this time. Moreover, the more marked alterations were obtained when the offspring were postnatally and pre + postnatally exposed to alcohol. Significant decreases were found in both T4 and T3 levels following postnatal exposure, except at the 20-25th day when a marked but transient increase in T4 levels was observed. These results indicate that alcohol exposure disturbs the hypothalamo-pituitary-thyroid axis, as measured by T3 and T4 hormone levels, mainly when the rats are exposed during the postnatal period.     

Effect of sex and age on serum aldosterone and thyroid hormones in the laboratory rat

Serum levels of aldosterone, tri-iodo thyronine (T3) and thyroxine (T4) were measured in male and female rats aged 3 months, 12 months, and 18 months. Female rats were found to have higher aldosterone and T3 levels, and lower T4 level than the male. No age-related change was observed in serum aldosterone in either sex. In contrast, serum T4 were found to decrease with age in both sexes while serum T3 showed an age-associated diminution in the male only. Serum testosterone was also measured in the male rats and was found to decline with age.