Exaggerated Circadian Variation in Basal Thyrotropin (TSH) and in the Dopaminergic Inhibition of TSH Release in Pathological Hyperprolactinemia: Evidence Against a Hypothalamic Dopaminergic Defect

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Abstract
In order to delineate more accurately the dopaminergic control of anterior pituitary function in normal subjects and in patients with pathological hyperprolactinemia, we investigated the nature of the circadian variation in the dopaminergic inhibition of TSH release in such subjects. Ten euthyroid women with hyperprolactinemia due to presumed PRL-secreting microadenomas (aged 18–60 yr) were compared with 11 normal, euthyroid women (aged 18–32 yr). Each received the dopamine receptor blocking drug domperidone (10 mg, iv) at 1100 and 2300 h (tests randomized and separated by at least 1 week). Blood was sampled 10, 20, 30, 45, and 60 min after drug administration. Normal women had a greater TSH response to domperidone and, hence, greater dopaminergic inhibition of TSH release at 2300 than at 1100 h (sum of TSH increments; mU/liter mean ± SE, 8.5 ± 1.3 vs. 4.8 ± 0.5, P < 0.01), whereas there was no difference in the dopaminergic inhibition of PRL release at each time of day. Hyperprolactinemic women also had a significantly greater TSH response to domperidone at 2300 than at 1100 h (42.0 ± 10.2 vs. 19.1 ± 2.8, P < 0.001). The hyperprolactinemic women had a greater TSH response to domperidone than normal women at each time of day studied (1100 h, 19.1 ± 2.8 vs. 4.8 ± 0.5, P < 0.001; 2300 h, 42.0 ± 10.2 vs. 8.5 ± 1.3, P < 0.001). The incremental PRL responses to domperidone were significantly less in hyperprolactinemic than in normal women and did not differ at each time of day. In conclusion, the circadian change in the dopaminergic inhibition of TSH secretion is specific for TSH and not PRL. This indicates that the dopaminergic control of TSH and PRL secretion can be dissociated in normal subjects. Second, hyperprolactinemic women with presumed PRL-secreting microadenomas had qualitatively normal but quantitatively exaggerated circadian pattern of dopaminergic inhibition of TSH release. These data argue against a hypothalamic dopaminergic defect in hyperprolactinemia and support the view that the established dopaminergic defect in the inhibition of PRL release is related specifically to PRL control and may well be at the anterior pituitary level.