Hypothyroid pituitary cells in culture: an analysis of thyrotropin and prolactin responses to dopamine (DA) and DA receptor binding

Biochemical diagnosis in prolactinomas: some caveats

Prolactinomas are the most frequently seen pituitary adenomas in clinical practice. A correct biochemical diagnosis of hyperprolactinemia is a prerequisite for further investigation but may be hampered by analytical difficulties as well as a large number of potentially overlapping conditions associated with increased prolactin levels. Suspicion should rise in patients whose symptoms and biochemical results do not match. Assay problems, macroprolactinemia, and high-dose hook effect are discussed as possible reasons for false positive or false negative prolactin levels. Physiological and pathological causes of hyperprolactinemia and their implications for interpreting prolactin results are reviewed.

Estimation of serum prolactin levels and determination of prevalence of hyperprolactinemia in newly diagnosed cases of subclinical hypothyroidism

Background:

Hyperprolactinemia is a common endocrine disorder involving hypothalamic–pituitary axis. Prolactin (PRL) secretion is stimulated by dopamine antagonism and thyroid-releasing hormone. Hyperprolactinemia has been reported in subclinical hypothyroidism (SCH) but results are markedly variable and studies on SCH are very few. The objective of this study was to find out prevalence of hyperprolactinema in newly diagnosed subclinical hypothyroid patients.

Materials and Methods:

In this cross-sectional study, serum PRL levels of 150 newly diagnosed subclinical hypothyroid patients were determined using electrochemiluminescence method.

Results:

Raised PRL levels were found in 18 (%) patients with SCH. There was positive correlation between serum thyroid-stimulating hormone and PRL levels. Prevalence of infertility was significantly higher with presence of hyperprolactinemia than normoprolactinemia in subclinical hypothyroid patients.

Conclusion:

Routine prolactin estimation and subsequent treatment is required in patients with subclinical hypothroidism.

Prevalence and predictors of hyperprolactinemia in subclinical hypothyroidism

BACKGROUND AND AIMS

Hyperprolactinemia has been reported in 0-57% of primary hypothyroidism. Data on hyperprolactinemia in subclinical hypothyroidism (ScH) is scant and inconsistent. This study aimed to determine the prevalence and predictors of hyperprolactinemia in ScH.

METHODS

Consecutive patients diagnosed to have normal thyroid function, ScH or overt primary hypothyroidism underwent serum prolactin, gonadotropins, testosterone and estradiol estimation. Patients with pregnancy, pituitary adenomas, secondary hypothyroidism, hyperthyroidism, comorbid states and drug-induced hyperprolactinemia were excluded.

RESULTS

From initially screened 4950 patients, hormonal data from 2848 individuals who fulfilled all criteria were analyzed. The occurrence of hyperprolactinemia (females:males) was highest in primary hypothyroidism (42.95%:39.53%) (n=192), followed by ScH (35.65%:31.61%) (n=770) and euthyroid individuals (2.32%:2.02%) (n=1886) (P<0.001). Hyperprolactinemia in ScH with TSH 5-7.5, 7.5-10 and >10mIU/L (females: males) was 25.56%:20.73%, 49.07%:50% and 61.43%:35.71% respectively (P<0.001). Significant positive correlation between TSH and prolactin was noted in ScH and primary hypothyroidism. In females, testosterone was lowest in patients with primary hypothyroidism. In males, serum estradiol was significantly higher, and testosterone significantly lower in men with ScH and primary hypothyroidism. Regression analysis revealed serum TSH followed by free T₄, to be best predictors of serum prolactin in both sexes.

CONCLUSION

Hyperprolactinemia is common in ScH, especially in those with TSH>7.5mIU/L. ROC analysis confirmed that TSH≥7.51mIU/L in females and ≥8.33mIU/L in males had a sensitivity of ≈50% with a very high specificity of >90% in detecting hyperprolactinemia. Prolactin screening may be warranted in ScH with TSH>7.5mIU/L, and may form an indication for treating ScH.

[Hyperprolactinemia: unusual association between peripheral hypothyroidism and microprolactinoma]

We report a rare case of hyperprolactinemia revealing the association between peripheral hypothyroidism and prolactin pituitary macroadenomas. The patient was a 43-year old woman, presenting with spontaneous bilateral galactorrhea over a period of 1 year. Hyperprolactinemia was confirmed and etiologic investigation revealed peripheral hypothyroidism secondary to autoimmune thyroiditis. Therapy consisted of administration of thyroid hormone, with clinical stabilization and hormonal normalization three months later. The evolution was marked by the persistence of hyperprolactinemia and galactorrhea. The diagnosis of microprolactinoma was objectified by pituitary MRI which showed microadenoma, justifying the administration of antidopaminergic therapy Six months later, the evolution was marked by normalization of prolactin levels and disappearance of pituitary microadenoma image.

Primary Hypothyroidism with Markedly High Prolactin

Secondary pituitary enlargement due to primary hypothyroidism is not a common manifestation. The loss of thyroxin feedback inhibition in primary hypothyroidism causes overproduction of thyrotropin-releasing-hormone (TRH), which results in secondary pituitary enlargement. TRH has a weak stimulatory effect on the lactotroph cells of the pituitary, so a mild to moderate increase in prolactin (PRL) levels is expected. We report the case of a 67-year-old female who presented with a large pituitary mass and a very high level of TSH in association with a significant rise in PRL level. In this case, diagnosing a sellar mass was challenging; it was difficult to distinguish between pituitary prolactinoma and primary hypothyroidism with secondary pituitary hyperplasia. Thyroid hormone replacement proved that this patient's hyperprolactinemia was due to hyperplasia of the pituitary gland. As such, making the correct diagnosis and initiating thyroid hormone therapy can prevent unnecessary treatment with dopamine agonists.

Pituitary hyperplasia: an uncommon presentation of a common disease

A 21-year-old woman presented with amenorrhea, bilateral galactorrhea and fatigue. Visual acuity and visual fields were normal. Laboratory examination demonstrated hyperprolactinemia. Magnetic resonance imaging (MRI) of the pituitary showed a 19×17×12-mm sellar mass with supra- and parasellar extension, causing compression of the pituitary stalk and optic chiasm. Further examinations confirmed mild hyperprolactinemia, strongly elevated TSH (>500 mU/l), low free thyroxine (FT₄), hypogonadotropic hypogonadism and secondary adrenal insufficiency. Hydrocortisone and l-T₄ replacement therapy was started. Three months later, the galactorrhea had disappeared, thyroid function was normalized and MRI revealed regression of the pituitary enlargement, confirming the diagnosis of pituitary hyperplasia (PH) due to primary hypothyroidism. Subsequently, the menstrual cycle returned and the hypocortisolism normalized. This case demonstrates that severe primary hypothyroidism may have an unusual presentation and should be considered in the differential diagnosis of pituitary enlargement associated with moderate hyperprolactinemia.

Evaluation of serum prolactin level in patients of subclinical and overt hypothyroidism

BACKGROUND

Prolactin secretion is controlled by prolactin inhibitor factor that is secreted from hypothalamus; factors like vasoactive inhibitory peptide (VIP) and thyroid releasing hormone (TRH) lead to increase in prolactin secretion. Hyperprolactinemia is a common condition that can result from a number of causes including hypothyroidism. Objective of the study was to determine correlation between serum levels of prolactin and thyroid hormones in euthyroid, subclinical and overt hypothyroid cases.

MATERIALS AND METHODS

Consecutive patients presenting for various thyroid related problems were segregated into two groups subclinical and overt hypothyroidism according to their diagnosis based on history and clinical examination, laboratory reports, inclusion and exclusion criteria. Newly diagnosed 75 patients in each group were finally enrolled. Similar number of age and sex matched controls were selected. All subjects filled a predesigned questionnaire for the evaluation of hypothyroid symptoms. Thyroid profile for T3, T4 (total and free), TSH and prolactin were determined in all the subjects and analyzed.

RESULTS

Prolactin elevation was found in 16 patients (21.33 %) with overt hypothyroidism, and in six patients (8%) with subclinical hypothyroidism. The control group and subclinical hypothyroid patients exhibited no significant difference in terms of total and free T3, total and free T4. For TSH and prolactin on the other hand, a statistically significant elevation was found in patients with overt hypothyroidism when compared with subclinical hypothyroidism; and in patients with subclinical hypothyroidism when compared to the controls. A significant statistical difference was observed between the two groups of hypothyroid patients for all hypothyroid symptoms except alopecia and hirsuitism.

CONCLUSION

The incidence of hyperprolactinemia in hypothyroidism was found to be higher when compared with normal controls. Serum prolactin assessment should be performed on all patients with hypothyroidism (overt and subclinical) before performing further tests.

Thyroxine-induced expression of pyroglutamyl peptidase II and inhibition of TSH release precedes suppression of TRH mRNA and requires type 2 deiodinase

Suppression of TSH release from the hypothyroid thyrotrophs is one of the most rapid effects of 3,3',5'-triiodothyronine (T(3)) or thyroxine (T(4)). It is initiated within an hour, precedes the decrease in TSHβ mRNA inhibition and is blocked by inhibitors of mRNA or protein synthesis. TSH elevation in primary hypothyroidism requires both the loss of feedback inhibition by thyroid hormone in the thyrotrophs and the positive effects of TRH. Another event in this feedback regulation may be the thyroid hormone-mediated induction of the TRH-inactivating pyroglutamyl peptidase II (PPII) in the hypothalamic tanycytes. This study compared the chronology of the acute effects of T(3) or T(4) on TSH suppression, TRH mRNA in the hypothalamic paraventricular nucleus (PVN), and the induction of tanycyte PPII. In wild-type mice, T(3) or T(4) caused a 50% decrease in serum TSH in hypothyroid mice by 5  h. There was no change in TRH mRNA in PVN over this interval, but there was a significant increase in PPII mRNA in the tanycytes. In mice with genetic inactivation of the type 2 iodothyronine deiodinase, T(3) decreased serum TSH and increased PPII mRNA levels, while T(4)-treatment was ineffective. We conclude that the rapid suppression of TSH in the hypothyroid mouse by T(3) occurs prior to a decrease in TRH mRNA though TRH inactivation may be occurring in the median eminence through the rapid induction of tanycyte PPII. The effect of T(4), but not T(3), requires the type 2 iodothyronine deiodinase.

The prevalence of hyperprolactinaemia in overt and subclinical hypothyroidism

The aims of this study were to: 1) determine the prevalence of hyperprolactinaemia in patients with newly diagnosed subclinical and overt hypothyroidism, and 2) investigate the change in PRL levels with treatment. In this observational study, patients with a new diagnosis of hypothyroidism in our endocrinology clinic were approached for participation, as were healthy controls. Patients with medical reasons for having elevated PRL levels, lactating and pregnant women were excluded from the study. No patient had kidney or liver disease. After examination to determine if clinical causes of PRL elevation were present, serum levels of thyrotropin (TSH), free thyroxine, free triiodothyronine and PRL were measured and correlation of PRL levels with the severity of hypothyroidism (overt or subclinical) was performed. Fifty-three patients (45 women, 8 men, mean age 45.3 ± 12.2 years) had overt hypothyroidism. One hundred forty-seven patients (131 women, 16 men, mean age 42.9 ± 12.6 years) had subclinical hypothyroidism. One hundred healthy persons (85 women, 15 men, mean age 43.9 ± 11.4 years) participated as controls. The same blood tests were repeated in patients after normalization of TSH levels with L-thyroxine treatment. PRL elevation was found in 36% of patients with overt hypothyroidism, and in 22% of patients with subclinical hypothyroidism. PRL levels decreased to normal in all patients after thyroid functions normalized with L-thyroxine treatment. In the hypothyroid patients (overt and subclinical) a positive correlation was found between TSH and PRL levels (r=0.208, p=0.003). PRL regulation is altered in overt and subclinical hypothyroidism, and PRL levels normalize with appropriate L-thyroxine treatment.

Effect of environmental and hypothalamic factors on thyrotropin secretion in the hypothyroid rat

1. Fourteen days after hypothyroidism was induced either by propylthiouracil (PTU) treatment or by thyroidectomy, the serum thyrotropin (TSH) responses to morphine (5 or 20 mg/kg bw), ether stress (30 min) and cold exposure (60 min) were compared with those in normal rats. 2. The decrease in serum TSH levels after morphine and ether stress found in the normal rats were abolished or much reduced respectively. 3. The increase in serum TSH in response to cold exposure and the diurnal rhythm of serum TSH (lower level at night) were also absent in the hypothyroid rat. 4. The stimulating effects of low dose of thyrotropin releasing hormone (TRH) and the inhibitory effects of somatostatin and apomorphine were completely abolished, while the stimulating effects of a high dose of TRH were much reduced in the hypothyroid rat. 5. These results indicate that in the hypothyroid rat the effect of a lack of negative feedback action of thyroid hormone predominates, and that hypothalamic factors are probably unimportant in the regulation of TSH secretion.

Regulation of adenohypophyseal messenger RNAs in female rats by age, hypothyroidism, estradiol and neonatal androgenization

Hormonal regulation of adenohypophyseal messenger ribonucleic acids (mRNAs) encoding preprotachykinin (PPT), prolactin (PRL) and thyrotropin beta subunit (TSH beta) was examined in juvenile and pubertal female rats. Hypothyroidism, initiated on day 2 (d2) or 22 (d22) of life, increased PPT and TSH beta mRNAs but decreased PRL mRNA 17 days later. Exogenous estradiol given for 3 days reduced PPT mRNA in pubertal (d38) but not juvenile (d18) euthyroid females; conversely, estradiol increased PRL mRNA on d18 but not d38. In hypothyroid females however, estradiol decreased PPT and TSH beta mRNAs at both ages and increased PRL mRNA in pubertal but not juvenile females. Thus, regulation of adenohypophyseal mRNAs by estradiol varies with age and thyroid status. In previous studies, adenohypophyseal tachykinins increased in male, but not female rats at puberty. This sex difference was not reproduced here by neonatal androgenization of females, suggesting that it is not mediated by hypothalamic sexual differentiation. However, PRL mRNA increased in androgenized females; this increase was prevented by ovariectomy, suggesting its medication by estradiol.

Dopamine, the dopamine D2 receptor and pituitary tumours

Dopamine plays an important role in the hypothalamic-pituitary axis where its major effects are to inhibit pituitary hormone secretion and cell division. Chronic dopamine deficiency has been postulated as a cause of pituitary tumour formation and several lines of evidence exist to suggest that a functional deficiency may develop as a result of defective dopamine receptor action. The available data suggest that a number of sites in the dopamine-D2 receptor-second messenger pathways may be implicated. These abnormalities are reflected in the variety of responses to dopamine and its agonists which have been observed in pituitary tumours both in the clinical situation and in cultured cells in vitro. Whilst it seems likely that the primary defect in pituitary tumour formation lies within the pituitary itself, the role of hypothalamic factors in facilitating tumour growth remains to be explored. Further studies of the dopamine receptor and its function will be of value not only in pathophysiological studies of human pituitary adenomas, but also in the development of new pharmacological agents to treat patients with these tumours.

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.

Regulation of GH and TSH release from hyperplastic and ectopic pituitaries: effects of dopamine in vitro

This study was undertaken to examine the consequences of prolonged removal of the pituitary from hypothalamic control and of estrogen-induced pituitary tumors on the susceptibility of GH and TSH release to regulatory influences of dopamine (DA). Adult male Fischer 344 rats were treated with transplants of female anterior pituitaries under the renal capsule or with Silastic capsules containing diethylstilbestrol (DES). Capsules with DES remained in place until the animals were killed (DES-IN) or were removed 7 weeks prior to sacrificing the rats (DES-OUT). Both pituitary grafts and DES caused the expected elevation in plasma prolactin and suppression of plasma GH and TSH levels. Basal GH release in vitro was not affected by exposure to DES in vivo but was reduced by transplantation of the pituitary to an ectopic site. Treatment with DA in vitro suppressed GH release from the in situ pituitaries of control, DES treated and grafted rats but increased GH release from the ectopic pituitaries. Basal release of TSH in vitro was reduced in the pituitaries of DES-IN and DES-OUT animals but was not affected by the presence of pituitary transplants. No detectable TSH was released from the ectopic pituitaries in the absence of DA. DA decreased TSH release from the pituitaries of control, DES-OUT and DES-IN rats but not from the in situ pituitaries of grafted rats. In contrast, DA produced an increase in TSH release from ectopic pituitaries. These results demonstrate that somatotrophs and thyrotrophs removed from the hypothalamic influences on subjected to direct and indirect effects of DES exhibit abnormal responses to DA. We suspect that prolonged absence of normal pituitary control leads to the development of regulatory mechanism of pituitary hormone release which are different from those operating under physiological conditions.

Dopamine D2 receptors are restricted to the prolactin cells in the rabbit pituitary gland: a combined autoradiographic and immunocytochemical study

The distribution of dopamine D2 receptors was studied using the autoradiographic technique on rabbit pituitary gland sections incubated in the presence of [3H]spiroperidol. D2 receptors were absent from the intermediate lobe as already reported. In the anterior lobe, the location of D2 receptors exactly overlapped that of the prolactin-producing cells revealed by immunocytochemistry on the sections used for autoradiography. No topographical relationships could be found between the D2 receptors and the TSH-producing cells which are known to be inhibited by dopamine in the rat.

Divergent dopaminergic regulation of TSH, free alpha-subunit, and TSH-beta in pituitary cell culture

TSH is a glycoprotein hormone composed of two nonidentical, noncovalently associated subunits, alpha and beta. We have previously shown in vivo that intrapituitary free alpha-subunit and intact TSH have divergent responses to hypothyroidism and thyroxine treatment, suggesting fundamental differences in their regulation. To explore this further, we exposed anterior pituitary cell cultures from rats previously rendered hypothyroid to thyrotropin releasing hormone (TRH), dopamine (DA), or TRH and DA and determined TSH, free alpha-subunit and TSH-beta responses. While positive or negative trends were noted at four hours, the most significant changes were observed at 24 and 48 hours. TRH increased media TSH at 24 hours to 180% of its basal value (P less than 0.01), with a comparable response at 48 hours. TRH also increased free alpha-subunit to 155% of the basal value (P less than 0.01) and TSH-beta to 145% of the basal value (P less than 0.01) at 24 hours. In contrast, DA produced concordant inhibition of TSH to 85% (P less than 0.05), free a-subunit to 42% (P less than 0.01), and TSH-beta to 53% (P less than 0.01) of the basal values at 24 hours. However, coincubation with both TRH and DA produced discordant responses: TSH was stimulated to 126% of the basal value at 24 hours (P less than 0.01), while both free alpha-subunit and TSH-beta fell significantly below the basal values (81% and 65% respectively, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Primary hypothyroidism presenting as amenorrhoea and galactorrhoea with hyperprolactinaemia and pituitary enlargement

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