Evidence for Dopaminergic Control of Circadian Variations in Thyrotropin Secretion

This study examines the influence of dopamine of recumbent circadian secretory patterns of plasma TSH. Seven euthyroid men were studied during a control (no medication) and a bromocriptine period (2.5 mg bromocriptine three times a day for 5 days). In the control period, a clear daily pattern in plasma TSH was evidenced, with highest levels occurring during the late evening presleep period and the fist few hours after the onset of nocturnal sleep. During the control period, recumbent 24-h plasma TSH was linearly correlated (gamma = 0.41; P less than 0.05) with clock time. Bromocriptine lowered the mean 24-h TSH from a control level of 2.1 +/- 0.2 to 1.1 microunits/ml. Further, bromocriptine eliminated the early nocturnal TSH surge and the correlation of plasma TSH with clock time. Thus, circadian variations in plasma TSH levels may be modulated, in part, by a dopaminergic mechanism. However, we cannot rule out a direct effect of bromocriptine on the anterior pituitary resulting in decreased late evening rises in plasma TSH.