RELATIONSHIPS BETWEEN THE CIRCADIAN RHYTHMS OF TSH, PROLACTIN AND CORTISOL IN SURGICALLY TREATED MICROPROLACTINOMA PATIENTS

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Abstract
Pharmacological doses of glucocorticoids inhibit TSH release both in vivo and in vitro and since the circadian rhythms of TSH and cortisol show a reciprocal relationship, the hypothesis has been advanced that changes in cortisol levels may be a primary determinant of circadian TSH changes. We have tested this hypothesis by studying the relationship between circadian cortisol and TSH rhythms in subjects before and during blockade with metyrapone. Seven patients were studied during their routine post‐operative assessment following selective transethmoidal adenomectomy for microprolactinomas. PRL levels were restored to normal (t‐testing of these data there was a small but significant reduction in the amplitude (P<0.05) and acrophase (P<0.05) of the TSH rhythm during metyrapone administration but no alteration in the circadian pattern of change. The normal circadian change in PRL levels which we observed in these patients was not altered by metyrapone treatment. It appears therefore that variation in endogenous cortisol is not the sole factor involved in the determination of the circadian TSH rhythm and that it does not influence circadian PRL changes in this group of subjects.