Effect of iopanoic acid on basal and thyrotropin-stimulated thyroid hormone levels in suckling rat pups

We proposed that basal and thyrotropin (TSH)-stimulated thyroid hormone levels of rat pups would be altered in the presence of iopanoic acid (IA), a radiographic contrast agent which competitively inhibits T4-to-T3 conversion, and that the nature of these changes would further depend upon the route of TSH administration in a manner distinct from that reported in adults. To test this hypothesis, litters from 24 Sprague-Dawley female rats were adjusted to 8 pups each. On day 5, 80 pups received IA (2.5 mg/100 g body weight) injections. On day 8, control and IA pups were further subdivided, and given bovine TSH (bTSH) either by subcutaneous injection or by intragastric gavage (to simulate milk-borne TSH intake), and then sacrificed 0, 1.5, or 3 hours later. We found significantly higher T4 and reverse-T3 (rT3) levels in IA-treated pups, but IA had no effect on basal or TSH-stimulated T3 levels attained, regardless of route of bTSH administration or time post-treatment. Our data demonstrate that the effects of IA on T4 and rT3 levels in the immature rat are comparable to those observed in adult rats and humans, but that the marked depression of T3 levels found in IA-treated adults does not occur in the 8-day old rat pup. We speculate that the IA-treated suckling pup's ability to sustain normal basal T3 levels and generate elevated T3 concentrations in response to TSH stimulation may reflect the activity during development of a T4-5'-deiodinase relatively resistant to competitive inhibition by this drug.     

Suckling ability and maternal prolactin levels in hypothyroid rats

Long-Evans rats and their offspring were made hypothyroid by addition of the antithyroid goitrogen 6-N-propylthiouracil (PTU) to the drinking water (0.1%) from the day of parturition. Serum concentrations of prolactin (PRL), thyroid-stimulating hormone (TSH) and thyroxine (T4) were determined by double radioimmunoassay (RIA). From the fifth postnatal day, body weight of PTU-treated pups was significantly lower than that of control rats, and a strikingly elevated serum TSH level and nondetectable amount of T4 were measured both in PTU-exposed mothers and their offspring at Day 10 postpartum. To test the youngs' suckling capability and the amount of maternal milk production, 10- and 15-day-old normal and PTU-treated pups were separated from their mothers for 4 hr in the morning and then reunited and allowed to suckle. Normal pups gained body weight at the end of both the first and second hour postreunion, while PTU pups gained only during the first hour and lost weight in the second hour of testing. When the pups were exchanged between normal and PTU mothers, opposite results were obtained, indicating that the reduced gain in hypothyroid rats was not due to impaired suckling capability, or insufficient sensory stimulation for milk secretion but to a decreased milk production of PTU mothers. In accordance with this, in lactating hypothyroid rats both the basal (presuckling) level and the suckling-induced rise of serum PRL were found significantly depressed.     

Impaired growth hormone secretion in neonatal hypothyroid rats: hypothalamic versus pituitary component

In 10-day-old rats made hypothyroid by giving dams propylthiouracil (PTU) in the drinking water since the day of parturition, simultaneous radioimmunoassay (RIA) determinations of basal and stimulated growth hormone (GH) secretion, hypothalamic GH-releasing hormone (GHRH)-like immunoreactivity (LI) content, immunocytochemical localization of somatotrophs, and hypothalamic GHRH-LI-positive structures were performed. The frequency of somatotrophs was also determined. One-day-old hypothyroid rats, whose mothers had been given PTU since the 14th day of pregnancy, were also used for comparison. In 10-day-old hypothyroid rats, pituitary and plasma GH levels and the number of somatotrophs were considerably lower and plasma TSH levels were significantly higher than those in age-matched control rats; however, GHRH-LI titers in the mediobasal hypothalamus and the morphology of GHRH-LI-positive structures were unaltered. In 1-day-old rats the only alteration present, in addition to elevated plasma TSH levels, was a clear-cut decrease in plasma GH levels. An acute challenge with GHRH (20 ng/100 g body wt, sc) or clonidine (15 micrograms/100 g body wt, sc) induced a clear-cut rise in plasma GH levels 15 min postinjection in 10-day-old control rats but failed to do so in age-matched hypothyroid rats. Both compounds failed to rise plasma GH in both hypothyroid and control 1-day-old rats. Taken together these data indicate that in neonatal and infant rats deprivation of thyroid hormones acts primarily to depress pituitary somatotroph function and that possible changes in GHRH-secreting structures represent a later postnatal event.     

The effect of beta-endorphin on basal and TRH-stimulated TSH release in conscious male rats

The inhibitory effect of beta-endorphin (EP) or other opioids on TSH secretion is, in contrast to their stimulating properties on PRL release, still a matter of debate. In the present study a dose of 1 microgram beta-EP injected intracerebroventricularly (IVT) in unstressed conscious male rats, though highly effective on PRL release, did not affect basal TSH levels, nor the TRH-induced TSH secretion. The previously reported inhibition of TSH release by opioids may therefore be an effect only seen when pharmacological doses are used.     

Impaired serum thyrotropin response to hypothyroidism in mice with disruption of neuromedin B receptor

Neuromedin B (NB), a neuropeptide highly concentrated in pituitary, has been proposed to be an inhibitor of thyrotropin (TSH) secretion. Previous study showed that mice with disruption of neuromedin B receptor (NBR-KO) have higher TSH release in response to thyrotropin-releasing hormone (TRH), although TSH seems to have decreased bioactivity. Here we examined in NBR-KO mice the response of TSH to thyroid hormone (TH) deprivation, obtained by methimazole treatment, or excess, obtained by acute and chronic TH administration. In response to hypothyroidism NBR-KO mice exhibited a lower magnitude increase in serum TSH compared to wild-type (WT) mice (1.7 vs. 3.3-times increase compared to euthyroid values, respectively, P<0.001). One hour after a single T4 injection (0.4 microg/100 g BW), WT and NBR-KO hypothyroid mice presented similar degree of serum TSH reduction (54%, P<0.05). However, 3 h after T4 administration, WT mice presented serum TSH similar to hypothyroid baseline, while NBR-KO mice still had decreased serum TSH (30% reduced in comparison to hypothyroid baseline P<0.05). T3 treatment of euthyroid mice for 21 days, with progressively increasing doses, significantly reduced serum TSH similarly in WT and NBR-KO mice. Also, serum T4 exhibited the same degree of suppression in WT and NBR-KO. In conclusion, disruption of neuromedin B receptor did not interfere with the sensitivity of thyroid hormone-mediated suppression of TSH release, but impaired the ability of thyrotroph to increase serum TSH in hypothyroidism, which highlights the importance of NB in modulating the set point of the hypothalamus-pituitary-thyroid axis at hypothyroidism.     

Triiodothyronine induces a transferable factor which suppresses TSH secretion in cultured mouse thyrotropic tumor cells

Modulation of TSH release from mouse thyrotropic tumor cells was studied. T₃ (1 nM) inhibited basal TSH release, while 6 nM T₃ blocked TRH-induced TSH release. Prior exposure of cells to actinomycin or cycloheximide prevented T₃ from suppressing basal and TRH-induced TSH release. The TSH-suppressive activity from T₃-treated cells was extracted and exposure of untreated thyrotropic cells to this material resulted in suppression of TSH release. The data suggest that T₃ suppression of TSH is mediated by formation of an inhibitory protein in thyrotropic cells.     

The use of intramuscularly administered methyl-TRH to evaluate the pituitary-thyroid axis.

The present study was carried out to evaluate the effectiveness of intramuscular administration of methyl-TRH, a potent analogue of thyrotropin-releasing hormone, for assessing pituitary reserve of TSH and prolactin and for distinguishing euthyroid, hypothyroid and hyperthyroid individuals. Serum samples were taken for 24 hours after intramuscular injection of methyl-TRH, 200 microgram, in 19 euthyroid subjects, 9 hypothyroid men and 9 hyperthyroid men. The mean serum prolactin and TSH concentrations were significantly elevated over baseline levels at 30 min in the euthyroid individuals and remained elevated for 3 to 4 hours. The serum TSH, T3 and T4 responses after intramuscular methyl-TRH in euthyroid subjects were clearly distinguishable from those of hyperthyroid and hypothyroid patients. Significant elevation of the serum T3 and T4 concentrations at 24 hours after intramuscular injection of methyl-TRH shows the sustained effect of this TRH analogue in euthyroid subjects.     

In vivo effects of gamma-aminobutyric acid and beta-alanine on thyrotrophin secretion in the normal and hypothyroid rat

The effect of pharmacological doses of two amino acids neurotransmitters, gamma-aminobutyric acid (GABA) and beta-alanine (beta-Ala), on thyrotrophin (TSH) secretion was studied in normal and hypothyroid (PTU-treated) male rats. Inhibition of TSH secretion was observed in normal rats treated with the drugs, 30 min after their administration. Hypothyroid animals responded only to GABA administration, decreasing their serum TSH at 30 min. Response to thyrotrophin-releasing hormone (TRH) after 15 min of drug administration was blunted in GABA injected animals, as compared to saline-injected controls. When TRH was injected at the same time as GABA and beta-Ala, the response was significantly lower than in controls. It is suggested that beta-Ala and GABA act at the pituitary by impairing the TSH response to TRH. The possibility that beta-Ala actions may be due to decreased GABA catabolism is considered, since beta-Ala administration increased GABA synaptosomal levels.     

Vitamin A-deficiency and thyrotropin secretion in the rat.

Basal serum TSH concentrations and TRH-induced TSH response were studied in control and in vitamin A-deficient rats at different times between the fifth week on diet (when growth of deficient animals was still normal) and the beginning of the weight plateau (as soon as growth of deficient animals had stopped). In deficient rats the TSH values were always lower than in the control rats. TRH injections (50 ng/100 g b.w.) in anaesthetized animals (amobarbital 1 mg/100 g b.w.) resulted in an approximately 12-fold increase in serum TSH levels within 6 minutes. The TSH levels remained elevated for at least 15 minutes and were similar in control and deficient rats. We hypothesize that the lower basal serum TSH concentrations are the result of a feedback mechanism triggered by an increase of serum free thyroxine (FT4) and free triiodothyronine (FT3).     

TSH and prolactin secretions in Hashimoto's thyroiditis following withdrawal of thyroid hormone therapy

Changes in the pituitary-thyroid axis in patients with Hashimoto's thyroiditis following withdrawal of thyroid suppressive therapy were analyzed. The group of patients with thyroid adenoma served as control (group I). Patients with Hashimoto's thyroiditis were divided into 2 groups on the basis of serum TSH levels 8 weeks after discontinuing the exogenous thyroid hormone (group II, less than 10 microunits/ml; group III, more than 10 microunits/ml). During treatment with L-T4(200 micrograms/day) or L-T3(50 micrograms/day), there was no significant difference in serum T4-I and T3 levels among the three groups. Following L-T4 withdrawal, basal serum TSH levels were higher at 2 to 8 weeks in groups II and III than in group I. Serum TSH response to TRH was greater at 4 to 8 weeks in groups II and III than in group I. Following L-T3 withdrawal, basal serum TSH levels were higher at 1 and 2 weeks in group II than in group I, while those of group III were consistently higher during the study. Higher TSH responses to TRH were observed at 1 to 8 weeks in groups II and III. Neither basal nor TRH-induced prolactin (PRL) secretion differed significantly among the three groups. We have demonstrated that pituitary TSH secretion in patients with Hashimoto's thyroiditis is affected more by withdrawal of thyroid hormone therapy than in patients with thyroid adenoma. In addition, the present findings suggest a difference between the sensitivity of thyrotrophs and lactotrophs in Hashimoto's thyroiditis after prolonged thyroid therapy is discontinued.     

Relationship of iodide-induced increase in TSH response to TRH to changes in serum thyroid hormones.

Large doses of iodide (500 mg three times a day) administered to normal men for 10--12 days caused a rise in basal serum TSH and a concomitant rise in the peak TSH response to TRH. The basal and peak levels of TSH were highly correlated (p less than 0.001). However, the iodide-induced rise in the peak TSH after TRH was poorly correlated with concomitant changes in serum thyroid hormones. Serum T3 wa not lower after iodide and, while serum T4 was somewhat lower, the fall in serum T4 was unexpectedly inversely rather than directly correlated with the rise in the peak TSH response to TRH. Thus, increased TSH secretion after iodide need not always be directly correlated with decreased concentrations of circulating thyroid hormones even when large doses of iodide are used. Clinically, a patient taking iodide may have an increased TSH response in a TRH stimulation test even though there is little or no change in the serum level of T3 or T4.     

Monocytes and platelets share the glycoproteins IIb and IIIa that are absent from both cells in Glanzmann''s thrombasthenia type I.

The possibility that thyroxine (T4) itself exerts the hormonal effect in vivo on the rat liver nuclear receptor was studied with the aid of iopanoic acid (IOP), an inhibitor of the conversion of T4 into tri-iodothyronine (T3). After administration of 2.4 micrograms of T4/100 g body weight to hypothyroid rats for 7 days, T4 and T3 concentrations in serum and in the liver nuclear non-histone protein (NHP) were all increased to the hyperthyroid range. Hepatic mitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD) activity and DNA content increased significantly. The equilibrium association constant (Ka) of the nuclear T3 receptor was unchanged and the maximal binding capacity (Cmax.) increased 1.4-fold. Simultaneous administration of IOP (5 mg/100 g body weight) to the rats given 2.4 micrograms of T4/100 g body weight completely blocked the conversion into T3. The serum T4 was even more increased, whereas the serum T3 decreased to the hypothyroid range. Although the NHP-bound T4 was at a concentration comparable with the rats given T4 alone, no NHP-bound T3 was detected. Yet the alpha-GPD activity was elevated 2.8-fold and the DNA content increased to the same extent as observed in the rats given T4 alone. The Ka and Cmax. of the nuclear receptor were significantly decreased. After administration of 48 or 480 micrograms of T4/100 g body weight for 3 days, serum T4 and T3 were markedly increased. The NHP-bound T3 was also increased, but no NHP-bound T4 was detected. The alpha-GPD activity was markedly elevated, but the DNA content was unchanged. The Cmax. per g of liver was increased, whereas the Ka remained unchanged. Simultaneous administration of IOP to these animals could not completely block the T4 conversion. The observed hormonal effects in the absence of nuclear T3 indicate that T4 possesses the intrinsic hormonal activities on the rat liver. T4 is less potent in induction of alpha-GPD activity but as potent in increment of hepatic DNA as T3. Although the binding site for T4 is not fully characterized, it appears to be acidic NHP. T4 is an active hormone, yet is also a prohormone of T3, offering the closest analogy with testosterone.     

Effects of substance P and neurotensin on growth hormone and thyrotropin release in vivo and in vitro

Conscious ovariectomized (OVX) rats bearing a cannula implanted in the third ventricle were injected with 2 μl of 0.9% NaCl containing varying doses of substance P (SP) or neurotensin (NT) and plasma GH and TSH levels were measured by RIA in jugular blood samples drawn through an indwelling silastic catheter. Control injections of physiologic saline iv or into the third ventricle did not modify plasma hormone levels. Intraventricular injection of SP or NT at doses of either 0.5 or 2 μg elevated plasma GH concentrations within 5 min and they remained elevated for 60 min. Third ventricular injection of similar doses of SP or NT had no effect on plasma TSH. An intermediate dose of 1 μg of SP or NT given iv had no effect on plasma GH but NT elevated plasma TSH. Incubation of hemipituitaries from OVX rats with varying doses of SP or NT did not alter GH release into the medium but TSH release was enhanced with NT at doses of 100 or more ng/ml of medium. It is suggested that SP acts centrally to stimulate growth hormone-releasing factor (GRF) or to inhibit somatostatin release and thereby enhance GH release and that NT acts directly on the pituitary to stimulate TSH release.     

Possible role of GH/IGF-1 in the ovarian function of adult hypthyroid rats

We have monitored estrous cycle and measured serum estradiol, GH, IGF-1, T4 and T3 levels in adult hypothyroid female rats which were divided into four groups: H group, hypothyroid rats without treatment; H-T4 group, hypothyroid rats injected daily with T4; HT4-PTU group, hypothyroid rats injected daily with T4 plus PTU (propylthiouracil), and H-T4-IOP group, hypothyroid rats injected daily with T4 plus IOP (iopanoic acid); Euthyroid rats (E group) were used as control. Our results indicate that the lack of sexual cycle in H animals was associated with lower values of estradiol, GH and IGF-1 in comparison to E group; the restoration of sexual cycle in H-T4 group was associated with values of estradiol, GH and IGF-1 higher than those of H group, whereas in H-T4-PTU and H-T4IOP groups the restoration was associated with higher values of GH and IGF-1 and values of estradiol similar to those of H group. These data could suggest a potential role of GH/IGF-1 axis, at least in part, in the lack of sexual cycle in H group and in the ovulation induction in H-T4, H-T4-PTU and H-T4-IOP groups.     

The effect of iopanoic acid on thyrotropin secretion in patients with cirrhosis of the liver

Ten patients with liver cirrhosis and six normal subjects were studied to evaluate the effect of iopanoic acid (IA) on thyrotropin secretion. A thyrotropin-releasing-hormone (TRH) test was performed before and 5 days after IA administration (single oral dose of 3 g). After IA administration, a significant increase in TSH response to TRH was observed in normal subjects. In cirrhotics, however, it did not significantly increase after IA administration. The serum T3 and T3/TBG ratio were significantly decreased and the serum T4 and T4/TBG ratio were increased after IA administration in normal subjects and cirrhotics. There was no significant difference in the % decrease in serum T3, % increase in serum T4 or other thyroid hormone parameters including TSH in IA induced TSH responders (R) and non-responders (NR). However, r-T3 before and after IA in R was higher than those in NR. The values for hepatic function tests such as serum albumin, prothrombin time, 45 minutes retention rate of bromsulphalein (BSP 45 min) and the cholinesterase (ChE) level in R were not different from those of NR. These results suggested that in cirrhotics, abnormal regulation of the hypothalamo-pituitary system might exist.     

Inhibitory effect of large doses of propylthiouracil and methimazole on an increase of thyroid radioiodine release in response to thyrotropin.

Thyroidal radioiodine release increased shortly after a single injection of small doses of PTU, while moderate doses of MMI produced a similar increase of thyroidal radioiodine release with a latency of 7-9 hr. Large doses of PTU and MMI failed to augment thyroidal radioiodine release for at least 29 to 34 hr after the initial administration of goitrogens, although plasma TSH increased significantly because of goitrogen administration. An increase of thyroid hormone release in response to exogenous TSH was depressed by PTU and MMI in rats and mice treated with T4. Since this depression of TSH action only continued for a short period in spite of continuous administration of goitrogens, and since final thyroidal radioiodine release rate was similar to that produced by small doses of PTU, the effects mentioned were not simply due to general toxic action of goitrogens. It is suggested that large doses of PTU and MMI not only block thyroid hormone synthesis but also interfere with the action of TSH on thyroid hormone secretion.     

Significant fluctuation of thyroid function in children with chronic lymphocytic thyroiditis

In order to investigate the degree of pituitary reserve of TSH secretion and the fluctuation of thyroid function in children with chronic lymphocytic thyroiditis, TSH response to TRH was examined in 42 patients, and the thyroid function was carefully followed up in two patients retrospectively and in four prospectively. Increased basal TSH levels were revealed in seven patients (16.8%), and an exaggerated response of TSH to TRH loading in 15 (35.8%). We retrospectively observed spontaneous recovery of thyroid function in two cases. In one of them, two episodes of a transient decrease in thyroid function over a period of several years were noted. Prospectively, low normal T4, elevated TSH and normal T3 were detected in two cases at the first visit. Thereafter, TSH levels decreased to the normal range and the exaggerated response of TSH to TRH became normal. In two other cases, typical transient hypothyroidism occurred during the observation period. These fluctuations lasted for only a few months, and concomitant changes in the size of the thyroid gland were observed. No signs or symptoms suggesting viral infection were noted during the study period. Nor were changes in titers of thyroid auto-antibodies detected. These results show that the secretion of TSH is exaggerated and the thyroid function is decreased in adolescents with chronic lymphocytic thyroiditis, but the thyroid function may fluctuate from euthyroid to hypothyroid within a short period. The causes of these changes, especially of the transient hypothyroidism remain to be classified.