Bromocriptine suppression of the thyrotrophin response to thyrotrophin releasing hormone

Effect of dopamine and a dopamine D-1 receptor agonist on pulsatile thyrotrophin secretion in normal women

The inhibitory effect of a pharmacological dose of dopamine and the specific dopamine D-1 receptor agonist fenoldopam on basal and pulsatile TSH secretion was investigated in normal women. The TSH response to fenoldopam and subsequent releasing hormone administration was also studied. Six women received placebo or dopamine infusion (4.0 micrograms/kg min) for 17 h. After 9 h, blood samples were collected every 10 min between 0800 and 1600 h for measurement of TSH. Eight women received 8-h (0900-1700 h) infusions of either fenoldopam (0.5 micrograms/kg min) or placebo. After 7 h of infusion 10 micrograms TRH, 5 micrograms GnRH and 25 micrograms CRF was given i.v. Blood samples were collected every 10 min. Dopamine infusion as well as fenoldopam infusion significantly reduced both mean basal TSH secretion and TSH pulse frequency compared with corresponding control infusions (P less than 0.05). However, while the effect on TSH pulsatility was comparable (P greater than 0.05), the percentage decrease in basal TSH levels after 16 h of dopamine infusion was 51 +/- 16% (mean +/- SD) and after 7 h of fenoldopam infusion 19 +/- 12% (P less than 0.05). Neither of the drugs affected TSH pulse amplitude and fenoldopam did not influence TRH-stimulated TSH release (P greater than 0.05). The results suggest that dopamine D-1 receptors are involved in modulation of TSH pulsatility probably at the hypothalamic level. It is argued that dopaminergic inhibition of basal TSH secretion and TSH pulsatility is predominantly regulated through dopamine D-2 receptors at the pituitary level, and through D-1 receptors at the hypothalamic level, respectively.

Effect of fenoldopam, a dopamine D-1 receptor agonist, on pituitary, gonadal and thyroid hormone secretion

The influence of fenoldopam, a dopamine (DA) D-1 receptor agonist, on basal and GnRH/TRH stimulated PRL, GH, LH, TSH, testosterone and thyroid hormone secretion was studied in nine normal men. All men received 4-h infusions of either 0.9% saline or fenoldopam at an infusion rate of 0.5 microgram/kg min, 12-16 ml/h, adjusted according to weight. After 3 h of infusion, 50 micrograms GnRH and 100 micrograms TRH was given i.v. Blood samples were collected every 15 min from 1 h before to 1 h after the infusion for a total of 6 h for measurements of PRL, LH, FSH, GH, TSH, testosterone, T4 and T3. The median PRL concentration increased significantly (P less than 0.01) to 128%, range 87-287, of preinfusion levels, compared to the decline during control infusion (85%, 78-114). Basal TSH levels declined significantly to 71% (60-91) during fenoldopam compared with 82% (65-115) during control infusion (P less than 0.05). Basal LH, FSH, GH and thyroid hormones were similar during fenoldopam and control infusions (P greater than 0.05). The LH response to GnRH/TRH was significantly (P less than 0.02) increased by fenoldopam infusion. Basal and stimulated testosterone concentration was lower during fenoldopam (P less than 0.01) infusion compared with control. Other hormones were similar after GnRH/TRH stimulation during fenoldopam and saline infusions. These results suggest that DA D-1 receptors are involved in the modulation of pituitary hormone secretion. We suggest that the effect of fenoldopam on PRL and TSH is mainly at the hypothalamic level. Regarding the effect on LH concentrations, an additional direct effect of fenoldopam on testosterone regulation can not be excluded.

Recovery of serum TSH and thyroid hormones after 3'isopropyl-3,5-diiodo-L-thyronine (DIIP) treatment: absence of inhibiting effect of bromocriptine on TSH secretion and evidence for autoregulation of serum T3 levels

Bromocriptine, because of its dopaminergic activity, could possibly inhibit TSH secretion. This hypothesis was tested in 7 normal male volunteers. Thyroid function was suppressed with the very potent thyromimetic analogue, 3'isopropyl-3,5-diiodo-L-thyronine (DIIP). We wanted to observe, after stopping this treatment, whether the return of serum T3, T4, rT3 and TSH values was influenced by concomitant bromocriptine treatment (5 mg/day). For comparison, the DIIP study was also performed without bromocriptine treatment. In both cases, serum T4 and serum T3, which had decreased by, respectively, 34% +/- 5% and 17% +/- 5%, returned over the same time interval to their initial values. Serum TSH and TRH-mediated TSH secretion were also unaffected by the bromocriptine treatment. DIIP does not interfere in the serum determination of T3, T4 and rT3. During suppression with DIIP serum T3 fell less than serum T4 and returned more rapidly to its initial concentrations. It is concluded that dopaminergic inhibition by bromocriptine, at this dose, is insufficient to alter normal thyroid function. In addition, during suppression and shortly thereafter there is a tendency for the serum T3 levels to be maintained.

Evidence for TSH autoregulation of thyroid carcinoma

Presumptive evidence for TSH dependency of thyroid hormone secretion from metastatic thyroid cancer is presented. In the patient described, the metastatic tumor was the only source of thyroid hormone production which was adequate enough to maintain the patient euthyroid. Tumor secretion of thyroid hormone appeared to regulate the TSH drive so that euthyroidism was maintained in the face of increasing tumor mass.

Correction by bromocriptine of hypothalamic dysfunction and post-prandial hypoglycaemic symptoms in a 31-year-old woman

A 31-year old female presented with recurrent episodes of post-prandial hypoglycaemic symptoms. Basal serum levels of ACTH, cortisol, GH, insulin and glucagon were normal. An adrenaline test demonstrated a normal peripheral response. An exercise test failed to produce ACTH, cortisol or FFA responses. Insulin (0.1 u/kg)-induced-hypoglycaemia failed to elevate serum ACTH, cortisol or GH. Metyrapone and ACTH tests were normal, demonstrating adequate hypophyseal and adrenal function. These findings suggested that the patient suffered from hypothalamic dysfunction. Bromocriptine (Parlodel, 7.5 mg/d for 5 weeks) resulted in an improved general condition, accompanied by a decrease in sugar consumption. Following treatment, FFA, ACTH and cortisol responses to exercise test were normal, as were ACTH, cortisol and GH responses to insulin-induced hypoglycaemia. It is concluded that bromocriptine may be useful in the treatment of post-prandial hypoglycaemic symptoms associated with hypothalamic dysfunction.

Nomifensine decreases the thyroid-stimulating-hormone response to thyrotropin-releasing-hormone in normal subjects

The Thyroid-stimulating-hormone (TSH) secretion has been studied in 12 normal euthyroid subjects (4 males, 8 females) after nomifensine (NOM) administration (200 mg po). NOM is a drug which activates dopaminergic neurotransmission at the Central Nervous System level. Blood samples were drawn every h for 4 h after NOM or placebo, respectively. At the 4th hour thyrotropin-releasing-hormone (TRH) was administered in bolus (200 micrograms iv), in both studies, and additional samples were collected for 90 min. The results show a moderate suppression (NS) of TSH and a clear-cut reduction in the secretory response to TRH after NOM administration (secretory area: TRH after placebo 723 +/- 132, TRH after NOM 400 +/- 83; p less than 0.01). The data appear to confirm that dopaminergic neurotransmission exerts an inhibitory role upon TSH secretion. The mechanism by which NOM induced dopaminergic activation leads to the suppression of TSH release after TRH has not yet been elucidated. An interference in TSH storage and/or in the post receptor mechanisms involved in TRH action might be hypothesized.

Dopaminergic modulation of TSH and its subunits: in vivo and in vitro studies

We have studied the effects of dopamine on the secretion of TSH and its subunits in vivo and in vitro. Four normal controls, seven patients with primary hypothyroidism, two patients with peripheral resistance to thyroid hormone (PRTH), and two patients with alpha-secreting pituitary tumours underwent a 3-h dopamine infusion (4 micrograms/kg/min). Serial blood samples were drawn for TSH, PRL, alpha, and TSH-beta subunit. In normal subjects, TSH fell from 2.1 +/- 0.9 (+/- SE) to 0.7 +/- 0.1 microU/ml (P less than 0.05), and alpha declined from 1.5 +/- 0.4 to 1.0 +/- 0.1 ng/ml (P less than 0.01). TSH-beta was at or slightly above the detection limits of the assay before and after dopamine. In hypothyroidism, basal serum TSH was 81 +/- 14 microU/ml. With dopamine, TSH fell to 35 +/- 8 microU/ml (P less than 0.001), while alpha decreased from 3.2 +/- 0.4 to 2.0 +/- 0.3 ng/ml (P less than 0.01). Serum TSH-beta also declined from 0.97 +/- 0.06 to 0.57 +/- 0.05 ng/ml (P less than 0.001). A similar fall in TSH and alpha was seen in the two patients with PRTH. In normals and hypothyroid patients, the percentage change in alpha concentration was significantly less than that observed for intact TSH. This is due presumably to the contribution of the gonadotrophs to the circulating alpha pool. TSH and TSH-beta were undetectable in the two pituitary tumour patients, and alpha declined only slightly in each patient after dopamine. The in vitro effects of dopamine were studied using cultured bovine anterior pituitary cells. Dopamine (10(-4)-10(-8) mol/l) did not change basal TSH, alpha, or TSH-beta release. However, dopamine at all doses significantly blunted TRH (10(-7) mol/l)-stimulated TSH and TSH-beta release, and blunted TRH-mediated alpha release at the two highest dopamine doses. These data suggest that dopamine modulates both TSH and TSH subunit secretion. These effects may be exerted directly at the level of the thyrotroph.

Effect of di-N-propylacetic acid (valproic acid) on the TSH response to TRH--a presumptive role for gamma aminobutyric acid

The effect of di-n-propylacetic acid (valproic acid), an inhibitor of gamma aminobutyric acid (GABA) transaminase, was studied with reference to its effect on the serum concentration of thyroid hormones, baseline serum TSH concentration and TRH stimulated TSH release, in seven normal controls and six patients with primary hypothyroidism. All volunteers took 250 mg of valproic acid administered orally, four times daily for 3 days. Baseline serum T4 and TSH concentrations were unaffected by valproic acid administration (p less than 0.05) while serum T3 concentrations fell in all volunteers (p less than 0.001). Serum T3 concentration (mean +/- SD) fell from 116.3 +/- 18 ng/dl to 101.7 +/- 15 ng/dl in the control group and from 94.2 +/- 47.9 ng/dl to 81.5 +/- 43.2 ng/dl in the hypothyroid group. Valproic acid produced a decline in stimulated serum TSH concentrations (delta TSH--maximum increment above baseline) in all controls and patients studied (p less than 0.01). delta TSH (mean +/- SD) declined from 16.1 +/- 4.7 microunits/ml to 10.5 +/- 5.8 microunits/ml in the control subjects and from 43.1 +/- 25.4 microunits/ml to 29.7 +/- 18 microunits/ml in the hypothyroid patients. Based on the data presented, it is postulated that GABA plays an inhibitory role either by acting directly on the pituitary gland inhibiting TSH release, or by inducing the secretion of a hypothalamic TSH-inhibitory factor. The data do not exclude a direct pharmacological effect of valproic acid on pituitary TSH release. Decrease in serum T3 following valproic acid may be due to peripheral mechanisms.

Altered dopaminergic regulation of thyrotrophin release in patients with prolactinomas: comparison with other tests of hypothalamic-pituitary function

This study was carried out to test the hypothesis that sustained hyperprolactinaemia in patients with prolactinomas stimulates hypothalamic dopaminergic activity via a short loop positive feedback effect of prolactin (PRL). The intensity of dopamine (DA) effects on the pituitary around the adenoma was evaluated by measuring thyroid stimulating hormone (TSH) responses to intravenous injection of domperidone (10 mg) a new DA receptor blocking drug that does not penetrate the blood-brain barrier. TSH responses have been compared with those of PRL to the same agent. Eight females with prolactinomas showed greater TSH release after domperidone than nine normal females (sum of TSH increments over 20 min 17.5 +/- 1.7 v. 8.9 +/- 1.5 mu/l, P less than 0.001) whilst PRL release was reduced (sum of PRL increments over 120 min 5.9 +/- 2.4 v. 21.8 +/- 3.8 mu/l x 10(-3), P less than 0.01). Amongst nineteen hyperprolactinaemic females with apparently normal pituitary fossae (plain skull X-ray), ten showed an exaggerated TSH response (delta TSH, 4.2 +/- 0.6 mu/l, range 2.5-9.0 mu/1) and reduced PRL response to domperidone, comparable with established tumor cases. In the remaining nine normal fossa hyperprolactinaemic females, the TSH and PRL responses to dopaminergic were similar to normal females. These results support the initial hypothesis and indicate the coexistence of a defect in the dopaminergic inhibition of PRL release and increased dopaminergic inhibition of TSH release in patients with prolactinomas. The presence of an exaggerated TSH response to DA antagonism in a euthyroid, radiologically normal (plain skull X-ray), hyperprolactinaemic patient is compatible with the presence of an autonomously-functioning, PRL secreting, pituitary microadenoma and the TSH changes seen in these patients after DA antagonist administration can be readily detected by sensitive TSH radioimmunoassay.

Pituitary function in normoprolactinaemic infertile men receiving bromocriptine

A double blind trial of bromocriptine 7.5 mg daily versus placebo was carried out in ten infertile men. Pretreatment basal plasma prolactin, thyroid stimulating hormone (TSH) testosterone and luteinizing hormone (LH) concentrations were normal, but plasma follicle-stimulating-hormone (FSH) was raised in four individuals. After 4 months treatment with bromocriptine a significant fall in plasma prolactin was observed (P less than 0.01), both under basal conditions and following thyroid stimulating hormone releasing hormone (TRH). Basal plasma gonadotrophin, testosterone and thyroid stimulating hormone (TSH) concentrations did not alter. No change in sperm density, volume or motility was noted. However an apparent fall in the peak plasma LH (but not FSH) response to gonadotrophin releasing hormone (LHRH) was observed in patients receiving bromocriptine. This reduction in plasma LH responsiveness was significant when compared with the baseline response (P less than 0.05) but failed to reach significance when compared with the placebo response. It is concluded that prolonged bromocriptine therapy in normoprolactinaemic men does not suppress FSH secretion, and any reduction in plasma LH responsiveness to LHRH is not accompanied by a significant fall in plasma testosterone.

Bromocriptine suppresses the thyrotrophin response to thyrotrophin releasing hormone during human pregnancy

Thyrotrophin (TSH) responses to 200 microgram of intravenous thyrotrophin releasing hormone (TRH) were measured in fifteen healthy women in normal early pregnancy before and at the end of a bromocriptine treatment of 5.0-7.5 mg daily for 1-2 weeks. Bromocriptine did not change the basal levels of TSH, triiodothyronine (T3) and thyroxine (T4) during pregnancy. Before the start of bromocriptine, TRH caused a significant TSH elevation from 12.8 +/- 0.5 muu/ml (mean +/- SE) to 21.2 +/- 1.9 muu/ml after 20 min. During bromocriptine intake, TRH caused a TSH elevation from 11.9 +/- 0.4 muu/ml to only 15.5 +/- 1.1 muu/ml which is significantly less (P less than 0.001) than before bromocriptine. Similarly, the mean maximal TSH increment of 8.4 +/- 1.5 muu/ml before bromocriptine was greater (P less than 0.001) than that of 3.8 +/- 60 muu/ml during bromocriptine intake. When women were retested with TRH before and during bromocriptine after legal abortion, bromocriptine did not change the basal levels of TSH, T3 and T4 or the TSH response to TRH. Therefore, the TSH inhibition caused by bromocriptine is specifically related to the pregnancy itself, but the mechanism for this inhibition remains unknown.

Effects of fibroblast growth factor and bromocriptine on the mitotic activity of the anterior pituitary gland in organ culture

Effects of fibroblast growth factor (FGF) and of bromocriptine, a dopaminergic receptor agonist, on the mitotic index in the organ-cultured anterior pituitary gland of the rat were investigated, using the colchicine metaphase-arrest technique. It has been found that FGF increases the mitotic index in the anterior pituitary explants. By contrast bromocriptine inhibits the mitogenic effect of FGF.

Chronic dopamine receptor stimulation using bromocriptine: failure to modify thyroid function

Administration of 2.5 mg bromocriptine (Parlodel), a dopamine agonist, on two occasions to six normal volunteers did not alter the plasma TSH response to an i.v. injection of 100 micrograms TRH, but significantly (P less than 0.01) blunted it after 200 micrograms. Chronic bromocriptine treatment (7.5--50 mg/day) of fifteen acromegalic subjects failed to influence basal plasma TSH or the response pattern to 200 micrograms TRH. The thyroxine Binding Index (TBI) and the levels of T3 and T4 were not modified by treatment. These results indicate that chronic dopaminergic therapy with bromocriptine does not alter thyroid function.