1. The effect of low dose dopamine infusion on the circadian rhythm of thyrotrophin (TSH), prolactin and Cortisol in a group of six healthy male volunteers is reported. Subjects were infused in random order with either saline (154 mmol/l NaCl solution; control) or dopamine (0.1 and 1 μg min−1 kg−1) between 21.00 and 01.00 hours, in random order.

2. The serum TSH profile was characterized by a maximal peak occurring at 23.00 hours and higher nocturnal than diurnal values. Superimposed on this are short term oscillations in serum TSH levels, typical of an ultradian rhythm.

3. The maximal peak in TSH, occurring at 23.00 hours, was abolished by dopamine infused at a rate of 1 μg min−1 kg−1, and was unaffected by the lower rate of dopamine infusion (0.1 μg min−1 kg−1).

4. The serum prolactin profile was characterized by a peak occurring soon after the onset of sleep (23.30–00.30 hours), which fell during the morning, and began to rise in late evening.

5. Low dose dopamine (0.1 μg min−1 kg−1) had a slight but insignificant effect with decreased prolactin levels at the end of the infusion whereas the higher dopamine dose was associated with significantly lower prolactin levels during and throughout the infusion. There was a rebound to levels significantly higher than control on cessation of the infusion. Cortisol levels were unaffected by dopamine.

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