Evidence for chicken GH as the only hypophyseal factor responsible for the stimulation of hepatic 5'-monodeiodination activity in the chick embryo

The influence of an intravenous injection of chicken growth hormone (cGH), a total chicken pars distalis (PD) extract, and a PD extract depleted of cGH by immunoadsorption was studied in the 18-d-old chick embryo. Plasma concentrations of triiodothyronine (T3), thyroxine (T4), and hepatic 5'-monodeiodination (5'-D) activity were measured. An injection of total PD extract raised plasma T3, T4, and 5'-D activity, whereas a PD extract depleted of GH only increased plasma T4. The amount of cGH present in the PD extracts, as measured by homologous cGH radioimmunoassay, increased T3 and raised liver 5'-D, but had no effect on plasma T4. The effect on liver 5'-D was more pronounced with cGH than with a total PD extract, whereas the effect on plasma T3 was somewhat less pronounced. It was concluded that cGH increased the peripheral conversion of T4 into T3 in the chick embryo, whereas a PD extract depleted of cGH was purely thyrotropic. The PD extract also seemed to have 5'-D-suppressing activity.     

Impaired peripheral T3 production but normal induced thyroid hormone secretion in the sex-linked dwarf chick embryo

Plasma concentrations of thyroxine (T4), triiodothyronine (T3) and chicken GH (cGH), together with hepatic 5'-monodeiodination (5'-D) activity, were measured in normal (Dw) and dwarf chick (dw) embryos at incubation d 18. An injection of 10 micrograms of ovine GH (oGH) raised plasma concentrations of T3 in Dw embryos after 1 and 2 h and stimulated hepatic 5'-D activity after 2 h. A non-specific increase in T4 was also observed after 1 h in Dw animals probably due to the heterologous nature of the injection. These effects were not observed in dw embryos. An injection of 1 microgram of TRH was able to increase cGH levels after 15 min in Dw embryos, whereas the the observed increase in the dw group was not significant. In Dw embryos, 0.01, 0.1 and 1 microgram of TRH increased plasma concentrations of T3 in a dose-dependent way, whereas in dw embryos, no reaction to the TRH injections was seen, except for the highest dose used. Contrary to this observation, T4 was increased to the same level in both Dw and dw embryos following TRH injections. An injection of 1 microgram of ovine CRH increased corticosterone after 0.5 h and elevated T3 and T4 after 2 h to the same extent in Dw and dw embryos. It is concluded that the thyrotrophic activities of TRH and oCRH and the corticotropic activity of oCRH do not differ between normal and sex-linked dwarf embryos. However TRH and GH were unable to stimulate the T4-T3 conversion in the liver of dw embryos, presumably due to the lack of hepatic GH receptors in these animals.     

Thyrotropin-releasing hormone (TRH) is not thyrotropic but somatotropic in fed and starved adult chickens.

Adult fed and starved Warren chickens, 2 yr of age, and approaching the end of the second laying year, were injected iv with 1 of the following products: 10 micrograms of thyrotropin releasing hormone (TRH); 100 micrograms of bovine thyrotropin (bTSH); 100 micrograms of ovine growth hormone (oGH); saline. The influence on plasma concentrations of thyroxine (T4), triiodothyronine (T3) or chicken GH (cGH) were followed. Prior to injection, it was clear from the control values that starvation for 3 d decreased plasma levels of T3 and increased cGH, whereas 7 d of fasting increased T4 and cGH. The plasma levels of cGH were elevated greater than 10-fold at 15 min following the TRH challenge in food-deprived chickens compared to a less than 4-fold increase in normal fed hens. This increase was followed by a rise in T3 after 1 h, which was also more pronounced in the starved animals, whereas T4 decreased or remained unaffected. Increases in T4 can, however, be obtained with 100 micrograms TSH in normal fed (2-fold) or starved animals (greater than 3-fold). Following injection of 100 micrograms oGH, a significant increase in T3 levels was observed which in fed animals was already present at 30 min, but the higher levels persisted for 1 and 2 h in fed and starved hens. At the same time, a decrease in T4 was observed in both groups of GH-treated chickens. It is concluded that TRH at the dose used is not thyrotropic but has a somatotropic effect and is responsible for the peripheral conversion of T4 into T3.     

Food intake after hatching inhibits the growth hormone induced stimulation of the thyroxine to triiodothyronine conversion in the chicken.

The effect of a single injection of 10 micrograms chicken GH on circulating thyroid hormones as well as in vitro liver 5'-monodeiodination (5'-D) activity was studied in posthatch chicks submitted to different feeding conditions. One group was normally fed after hatching, a second group was only fed after three days and a third group was food deprived after 2 days of feeding. Combination of all results indicates that the start of food intake abolishes the stimulatory effect of a GH injection on circulating T3 and liver 5'-D activity. Food deprivation after a period of food intake restores the GH effect on plasma T3 but not on liver 5'-D.     

Significance of serum thyrotropin and plasma dopamine concentration in the regulation of thyroid function in elderly subjects

In our previous study, we observed a tendency towards an age-related increase in the serum thyrotropin (TSH) concentration. Regulatory mechanisms of TSH secretion in elderly subjects were studied. In 43 elderly subjects, serum TSH did not correlate significantly with serum T4, T3 free T4 or rT3. Further, those with increased TSH (greater than 5 mU/l, 9 subjects) did not overlap with those with low T3 (less than 0.92 nmol/1, 8 subjects). Increases in serum TSH were not associated with the presence of circulating anti-thyroid autoantibodies. A TRH test using a 500 micrograms single bolus injection was performed in 15 subjects. TSH response (basal: 1.92 +/- 1.42 (s.d.) mU/1, peak: 11.25 +/- 5.33 mU/1, sigma: 26.74 +/- 12.89 mU/1, respectively) did not differ significantly from that of younger subjects. T3 response after TRH varied greatly and a close correlation was observed between basal T3 and peak T3 (r = 0.86), and also between peak T3 and delta T3 (r = 0.81). A significant correlation was observed between sigma TSH and basal T3 (r = 0.60). Neither plasma cortisol, epinephrine nor norepinephrine concentrations showed any significant correlation with basal and TRH-stimulated TSH or T3 concentrations. However, the plasma dopamine concentration correlated significantly with sigma TSH (r = 0.60) and basal T3 (r = 0.52), respectively. In conclusion, the increase in serum TSH observed in elderly subjects was felt to represent a physiological adaptation to maintain serum T3. Low T3 subjects appear to have a disturbance in this mechanism, with decreased TSH and T3 response to TRH stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thyroid hormone response to thyrotrophin releasing hormone (TRH) in the sex-linked dwarf chicken

The effect of an injection of thyrotrophin releasing hormone (TRH) on plasma levels of thyroid hormones was studied in dwarf and normal Rhode Island Red chickens with similar genotypes other than for the sex-linked dwarf gene dw. The sex-linked dwarf chickens had different plasma iodothyronine levels from control normal chickens: high thyroxine (T4), low triiodothyronine (T3) and similar reverse T3 (rT3) levels. The injection of TRH (10 micrograms/kg) in 5-day- and 5-week-old normal chickens increased the plasma T4 within 30 min without a significant increase in T3, whereas the injection of TRH in 11-and 26-week-old normal chickens increased plasma T3 60 min later. In dwarfs the response of T4 to TRH was the same as that in normals but no increased T3 response was observed. The plasma level of rT3 was not influenced by the TRH injection in either strain. These results suggest that although in the sex-linked dwarfs thyroidal response to exogenous TRH is similar to that of normals, the dwarf gene dw inhibits the conversion of T4 to T3 in peripheral tissues without any inhibitory effect on rT3 production.     

Influence of D-thyroxine on plasma thyroid hormone levels and TSH secretion.

Triiodothyronine (T3), thyroxine (T4), basal TSH and TSH after stimulation with TRH were determined in healthy subjects and patients treated with D-thyroxine (DT4). After a dosage of 6 mg DT4 the D/L T4 plasma concentration rose about 4-fold 4 hours after application and was only moderately elevated 14 hours later. To achieve constantly elevated T4 levels 3 mg DT4 were applied in the further experiment every 12 hours. The D/L T4 plasma concentration rose 2.5-4-fold and there was a small but significant increase of the D/L T3 plasma concentration. 74 hours after onset of treatment basal TSH was below detectable limits and the increase of TSH 30 min after injection of 200 mug TRH (TRH test) was only about 15% compared to zero time. The time course of TSH suppression was investigated after treatment with DT4 and LT4 (single dosage of 3 mg). TRH-tests were performed before, 10, 26, 50 and 74 hours after the first dosage of D or LT4. There was no difference in the time course of basal TSH and TSH stimulated by TRH. In 10 patients on DT4 long-term therapy, basal and stimulated TSH were found to be below the detectable limits of 0.4 mug/ml. Our results show that (1) plasma half-life of DT4 is less than 1 day, (2) TSH suppression after D and LT4 treatment is very similar, and (3) in patients on long-term DT4 treatment, TSH plasma concentration is below detectable limits even after stimulation with TRH.     

The thyroid reserve in children with chronic lymphocytic thyroiditis revealed by the thyrotropin-releasing hormone and thyroid-stimulating hormone tests

Serum thyroid hormone and TSH concentrations were measured before and after the administration of TRH (10 micrograms/kg body weight) and bovine TSH (10 IU) in 14 children with chronic lymphocytic thyroiditis. The TRH test showed that the responsiveness of TSH was positively correlated with the basal TSH (P less than 0.001) and inversely with the increase in serum thyroid hormones, for delta T3 (P less than 0.05) and for delta T4 (P less than 0.001). Overall, the patients had significantly lower mean values for basal T4, but not for T3. The TSH test revealed that the delta T3 was positively correlated with delta T4 (P less than 0.05). delta T3 after TSH administration was positively correlated with it after TRH (P less than 0.05). The patients were divided into three groups on the basis of their peak TSH values after TRH administration. In Group 1 (peak value below 40 microU/ml; N = 5); T3 increased significantly after TRH and TSH administrations (P less than 0.05 and P less than 0.025, respectively). In addition, delta T4 was significant after TSH administration. In Group 2 (peak TSH above 40 and less than 100 microU/ml; N = 6); only delta T3 after TRH was significant (P less than 0.05). In Group 3 (peak TSH above 100 microU/ml; N = 3); the response of thyroid hormones was blunted. Thus, the thyroid hormone responses to endogenous TSH coincided with that to exogenous TSH, and the exaggerated TSH response to TRH indicates decreased thyroid reserve.     

The pituitary-thyroid axis in patients with pituitary disorders

The pituitary-thyroid axis of 12 patients, exposed to transsphenoidal pituitary microsurgery because of nonfunctioning adenomas (6), prolactinomas (3) and craniopharyngioma (1), or to major pituitary injury (1 apoplexy, 1 accidental injury), was controlled more than 6 months following the incidents. The patients did not receive thyroid replacement therapy and were evaluated by measurement of the serum concentration of thyroxine (T4), 3,5,3'-triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3), T3-resin uptake test and thyrotropin (TSH, IRMA method) before and after 200 micrograms thyrotropin releasing hormone (TRH) iv. The examination also included measurement of prolactin (PRL) and cortisol (C) in serum. Apart from 1 patient with pituitary apoplexy all had normal basal TSH levels and 9 showed a significant TSH response to TRH. Compared to 40 normal control subjects the 12 patients had significantly decreased levels of T4, T3 and rT3 (expressed in free indices), while the TSH levels showed no change. Five of the patients, studied before and following surgery, had all decreased and subnormal FT4I (free T4 index) after surgery, but unchanged FT3I and TSH. The levels of FT4I were positively correlated to both those of FT3I and FrT3I, but not to TSH. The TSH and thyroid hormone values showed no relationship to the levels of PRL or C of the patients exposed to surgery. It is concluded that the risk of hypothyroidism in patients exposed to pituitary microsurgery is not appearing from the TSH response to TRH, but from the thyroid hormone levels.     

Studies on the effect of neonatal hypermetabolism on hypothalamo-pituitary-thyroid axis in adult rats.

Neonatal rats which had received a daily injection of 50 microgram of 2,4-dinitrophenol (DNP) or 30 microgram of L-thyroxine (T 4) for 7 days beginning on the day of birth were compared as to the late effect of the hypothalamo-pituitary-thyroid axis with the neo saline control. Neo DNP rats and neo T 4 rats revealed the retardation of growth compared with neo saline rats. The plasma level of TSH in both groups presented its low response following TRH administration. Furthermore, plasma TSH levels following the challenge of PTU were depressed in both neo DNP and neo T 4 rats compared with neo saline control rats. A small dose of T 4 injection, however, did not bring any difference on plasma TSH levels between neo T 4 and neo saline control rats while neo DNP rats showed a little blunted response of pituitary compared with neo T 4 and neo saline rats. Pituitary contents of TSH in neo T 4 rats decreased, but not in neo DNP rats. These results suggest that neonatal hypermetabolism causes the hypofunction of pituitary-thyroid axis through adult life and that the alteration of hypothalamus may be more obvious in neo T 4 rats than in neo DNP rats.     

Galactorrhea in subclinical hypothyroidism

Galactorrhea was found in 5 patients with subclinical hypothyroidism. The galactorrhea consisted of the discharge of a few drops of milk only under pressure. Serum T4 was in the lower level of the normal range, but serum T3 was normal (T4: 6.3 +/- 1.2 micrograms/dl, T3: 113 +/- 7 ng/dl). Basal serum TSH and PRL were slightly increased only in 2 and 1 cases, respectively. The PRL responses to TRH stimulation were exaggerated in all cases, although the basal levels were normal. An enlarged pituitary gland was observed in 1 patient by means of CT scanning. All patients were treated by T4 replacement. In serial TRH tests during the T4 replacement therapy, the PRL response was still increased even when the TSH response was normalized. Galactorrhea disappeared when the patients were treated with an increased dose of T4 (150-200 micrograms/day). Recurrence of galactorrhea was not observed even though replacement dose of T4 was later decreased to 100 micrograms/day in 4 cases. In patients with galactorrhea of unknown origin, subclinical hypothyroidism should not be ruled out even when their serum T4, T3, TSH and PRL are in the normal range. The TRH stimulation test is necessary to detect an exaggerated PRL response, as the cause of the galactorrhea. To differentiate this from pituitary microadenoma, observation of the effects of T4 replacement therapy on galactorrhea is essential.     

Effect of non aromatizable androgens on LHRH and TRH responses in primary testicular failure

We have assessed the gonadotropin, TSH and PRL responses to the non aromatizable androgens, mesterolone and fluoxymestrone, in 27 patients with primary testicular failure. All patients were given a bolus of LHRH (100 micrograms) and TRH (200 micrograms) at zero time. Nine subjects received a further bolus of TRH at 30 mins. The latter were then given mesterolone 150 mg daily for 6 weeks. The remaining subjects received fluoxymesterone 5 mg daily for 4 weeks and 10 mg daily for 2 weeks. On the last day of the androgen administration, the subjects were re-challenged with LHRH and TRH according to the identical protocol. When compared to controls, the patients had normal circulating levels of testosterone, estradiol, PRL and thyroid hormones. However, basal LH, FSH and TSH levels, as well as gonadotropin responses to LHRH and TSH and PRL responses to TRH, were increased. Mesterolone administration produced no changes in steroids, thyroid hormones, gonadotropins nor PRL. There was, however, a reduction in the integrated and incremental TSH secretion after TRH. Fluoxymesterone administration was accompanied by a reduction in thyroid binding globulin (with associated decreases in T3 and increases in T3 resin uptake). The free T4 index was unaltered, which implies that thyroid function was unchanged. In addition, during fluoxymesterone administration, there was a reduction in testosterone, gonadotropins and LH response to LHRH. Basal TSH did not vary, but there was a reduction in the peak and integrated TSH response to TRH. PRL levels were unaltered during fluoxymesterone treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of anti-thyrotropin-releasing hormone anti-serum treatment during the neonatal period on the development of rat thyroid function

Effects of anti-thyrotropin-releasing hormone (TRH) anti-serum treatment during the neonatal period on the development of rat thyroid function were studied. On postnatal days 2 and 4, rats were administered anti-TRH anti-serum ip, and they were serially decapitated at the 4th, 8th and 12th week after birth. TRH, thyrotropin (TSH), thyroxine (T4) and 3,3',5-triiodothyronine (T3) were measured by radioimmunoassay. Immunoreactive TRH (ir-TRH) in the hypothalamus did not change significantly after anti-TRH anti-serum treatment, and plasma ir-TRH tended to decrease. The plasma ir-TRH and TSH responses to cold were significantly inhibited. The plasma TSH response to TRH was also significantly inhibited. The plasma basal TSH levels were significantly lower than in controls. The plasma T4 and T3 levels were found to be lower than those in the controls. Findings suggested that treatment with anti-TRH anti-serum during the neonatal period disturbed the development of rat thyroid function, inhibiting TRH release and altering thyrotroph sensitivity to TRH.     

Influence of chronic thyroxine treatment on plasma hormone and metabolite concentrations and on responses to insulin, glucagon and thyrotrophin releasing hormone in adult sheep

Four adult sheep fed twice daily were given daily subcutaneous injections of saline for four weeks, followed by a similar period of daily L-thyroxine (T4) injection (1 mg/day). T4 treatment increased basal plasma concentrations of T4, triiodothyronine (T3), insulin and glucose, together with T3-uptake and the free thyroxine index, while cholesterol and urea concentrations decreased. T4 treatment reduced the rise in prolactin levels after the morning meal. Thyrotrophin releasing hormone (TRH) injection increased plasma T3 only in the control period and T3-uptake only in the T4 treatment period. T4 treatment did not affect the prolactin response to TRH injection or the insulin and glucose responses to glucagon injection. The increase in insulin concentrations after insulin injection and the secondary hyperglycaemia following initial insulin-induced hypoglycaemia were reduced by T4 treatment.     

Long-term intramuscular administration of thyrotropin-releasing hormone tartrate in patients with cerebrovascular disease: effects on the pituitary-thyroid axis.

We studied the effects of long-term (30 days) refracted daily intramuscular administration of 4 mg TRH tartrate (TRH-T) on the pituitary-thyroid axis in 20 euthyroid patients affected by cerebrovascular disease (CVD). All subjects were assayed for T4, T3, FT4, FT3, TSH and TBG plasma levels before treatment (D0), after 15 and 30 treatment days (D15, D30), and after a 15-day washout (D45). In addition, TSH response to 200 micrograms intravenous TRH was assessed at D0, D30 and D45. We observed a significant increase in T4, FT4 and FT3 levels in the face of decreased TSH concentrations. A blunted TSH response to TRH bolus persisted at D30. These data demonstrate that the down-regulation mechanism may be partially overcome in vivo when thyrotrophs are chronically exposed to pharmacological TRH-T doses and that TSH pattern is mainly due to the negative feedback of thyroid hormones, even though pituitary TSH reserves may become depleted. Furthermore, prolonged TRH-T administration does not produce hyperthyroidism in euthyroid CVD patients.     

Effect of active and passive immunization with TRH on plasma TSH response to propylthiouracil.

The role of thyrotropin-releasing hormone (TRH) in the secretion of TSH from the anterior pituitary was investigated in rats by active and passive immunization with TRH. The plasma TSH response to propylthiouracil (PTU) in TRH-bovine serum albumin (BSA)-immunized rats was significantly lower than that of BSA-immunized or non-immunized rats. Similarly, the increased plasma TSH level following PTU treatment was significantly suppressed after iv injection of antiserum to TRH. However, the decline in plasma TSH levels was not complete. The results of the present study indicate, at least in part, the physiological significance of endogenous TRH in the regulation of pituitary TSH secretion.     

A decreased capacity of hepatic growth hormone (GH) receptors and failure of thyrotrophin-releasing hormone to stimulate the peripheral conversion of thyroxine into triiodothyronine in sex-linked dwarf broiler hens

The effect of two different doses of thyrotrophic releasing hormone (TRH) upon the plasma levels of growth (GH) and thyroid hormones in both sex-linked dwarf (dw) and normal (Dw) broiler hens was determined. In normal hens, 1.5 and 24 microg TRH/kg increased the GH plasma concentrations after 15 min. Plasma concentrations of T3 increased significantly 1 h after TRH injection, whereas T4 concentration decreased after 2 following injection of 24 microg/kg TRH. In dwarf hens both doses of TRH increased the plasma concentrations of GH and the GH response lasted longer. However, TRH was ineffective in raising T3 and T4 levels. Saline-injected dwarf birds showed no differences in plasma T4 and T3 levels in comparison with normal hens. A smaller number of hepatic cGH receptors was found in dwarf hens, whereas the affinity of the hepatic GH receptor was not influenced by the genotype. It is concluded that the sex-linked dwarf broiler hen is unable to respond to a TRH-induced GH stimulus probably because of a deficiency in hepatic GH receptors resulting in a failure to stimulate the T4 to T3 converting activity.     

Adrenal inhibition of corticotropin-releasing hormone-induced thyrotropin release: a comparative study in pre- and posthatch chicks

Recent evidence indicates that corticotropin-releasing hormone (CRH) acts as a potent stimulator of thyrotropin (TSH) release in the chicken. In this study adrenal and thyroidal feedback mechanisms were studied. Administration of corticosterone 30 min prior to an ovine CRH (oCRH) challenge diminished the in vivo sensitivity of thyrotrophs to oCRH in 19-day-old chicken embryos (E19) (20 micrograms corticosterone; 2 micrograms oCRH) but not in 8-day-old chickens (C8) (40 micrograms corticosterone; 4 micrograms oCRH). At both ages studied, corticosterone (0.01 and 1 microM) did not alter the in vitro TSH response to oCRH (100 nM) indicating that an indirect mechanism is involved at the embryonic stage which is no longer present in posthatch chickens. In vitro, 3,5,3'-triiodothyronine (T3) pretreatment (0.01 and 1 microM) resulted at both ages studied in a dose-dependent drop in the in vitro oCRH-induced TSH release. As recorded previously, corticosterone treatment provoked a rise in plasma T3 in embryonic but not in posthatch chickens. The presence of an indirect adrenal feedback mechanism in chicken embryos may therefore be linked to the increase in plasma T3 which will alter the sensitivity of thyrotrophs to hypothalamic releasing factors. In conclusion, corticosterone does not directly modulate the responsiveness of thyrotrophs to CRH, but its feedback mechanism may be dependent on the evoked increase in plasma T3 which is only present in embryonic chickens. Corticosterone may in this regard play an essential role during embryonic development by coordinating thyroidal feedback mechanisms at the level of the chicken pituitary.     

Potentiation by indomethacin of TRH-induced TSH secretion in the rat.

We have studied the effect of two inhibitors of prostaglandin synthesis on the basal and TRH-stimulated plasma TSH levels in the rat. Animals were injected sc daily with indomethacin 3 mg/0.5 ml) or aspirin (16--30 mg/0.5 ml) for 3 days. The plasma T4 and T3 were consistently lower in the indomethacin or aspirin groups than in the controls, while the basal TSH levels did not change. Indomethacin treatment significantly potentiated the TSH response to synthetic TRH (20 ng. iv) in intact and thyroidectomized rats. The pituitary TSH content was markedly increased by indomethacin, while hypothalamic TRH content did not change. In contrast, aspirin inhibited the TSH response to TRH in intact rats, when pituitary TSH content decreased significantly. No potentiation by aspirin of TRH-stimulated TSH response in the thyroidectomized rats was observed. The increased sensitivity of plasma TSH response to exogenous TRH in the indomethacin group is presumably due to higher pituitary TSH content than in the controls. The action of indomethacin appears to be mediated, at least in part, at the pituitary level. In addition, there is a dissociation between the action of indomethacin and the action of aspirin in the TSH response to TRH.