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Circadian Responsiveness of the Hypothalamic-Pituitary Axis

Department of Endocrinology, Postgraduate Institute of Medical Education and Research Chandigarh, India

Reprints: G. K. Rastogi, Chief of Endocrinology, Postgraduate Institute of Medical Education and Research, Chandigarh-160011, India.

Five healthy men 25–38 years old were subjected to simultaneous composite intravenous stimulation tests of insulin hypoglycemia (0.1 U/kg), thyrotropin-releasing hormone (TRH, 100 µg), and luteinizing hormone-releasing hormone (LHRH, 50 µg) at weekly intervals to study the circadian responsiveness of the hypothalamic-adenohypophyseal axis at 0600, 1200, 1800, and 0000 hours. Blood sugar (BS), LH, follicle-stimulating hormone, TSH, prolactin, cortisol (C), growth hormone, and testosterone (T) levels were estimated before and after the administration of drugs. Comparisons were made between basal and values (difference between basal and peak or nadir levels) at different tests. Significant circadian variations in BS, GH, C, and, to a lesser extent PRL, responses were observed; 0600 h basal and BS values were the lowest, the A BS was highest at 0000 h accompanied by maximal hypoglycemic symptoms; the values of both C and GH were significantly higher at 0600 h and 0000 h; highest mean PRL was observed at 0600; at 1800 h the basal plasma PRL level was maximum but the PRL was lowest. Plasma TSH, LH, and FSH responses did not show significant circadian variations. These results suggest that circadian variations are evident when stimuli act through central or hypothalamic mechanisms; however, direct stimulation of the adenohypophysis resulted in identical responses at different periods tested.

Supported in part by W.H.O. research contract H9/181/72/E and the P.G.I, research fund.

Received July 16, 1975.

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