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Pulsatile Secretion of Thyrotropin during Fasting: A Decrease of Thyrotropin Pulse Amplitude^*

Departments of Intensive Care (J.A.R.) and Endocrinology (R.A., W.M. W.), Academisch Medisch Centrum, University of Amsterdam 1105 AZ Amsterdam, The Netherlands
The Department of Clinical Endocrinology, Medizinische Hochschule (G.B., K.P.) Hannover, West Germany

Address all correspondence and requests for reprints to: J. A. Romijn, M.D., Department of Intensive Care, C3W, Academisch Medisch Centrum, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands.

The effect of fasting on circadian and pulsatile TSH secretion was investigated in eight healthy subjects (four men and four women in the follicular phase). Each subject was studied twice, once during 24 h with normal food intake and once during the last 24 h of a 60-h fast. Blood was sampled every 10 min during 24 h for measurement of TSH by a sensitive immunoradiometric assay.

Fasting induced a decrease in plasma T₃ [1.73 ± 0.06 vs. 1.36 ± 0.04 nmol/L; P < 0.01 (mean ± SE), control period vs. fasting] and thyroglobulin (52 ± 8 us. 35 ± 7 pmol/L; P < 0.001) and an increase in plasma rT₃ (0.30 ± 0.06 vs. 0.44 ± 0.09 nmol/L; P < 0.02). Plasma T₄, thyroid hormone binding index, and free T₄ were not statistically different in both periods. The mean plasma 24-h TSH concentration was lower during fasting than in the control period (2.0 ± 0.3 vs. 1.0 ± 0.2 mU/L; P < 0.005). This was associated with a decrease in mean TSH pulse amplitude during fasting (Desade program: 0.6 ± 0.1 us. 0.3 ± 0.1 mU/L; P < 0.01; Cluster program: 0.5 ± 0.1 vs. 0.2 ± 0.1 mU/L; P < 0.05), whereas TSH pulse frequency during fasting was unchanged (Desade program: 8.4 ± 0.9 vs. 9.8 ± 0.8 pulses/24 h; Cluster program: 9.5 ± 0.5 us. 7.9 ± 0.9 pulses/24 h). There was a highly significant correlation between the mean 24-h TSH concentration and the mean TSH pulse amplitude during both the control period and fasting.

Although the decrease in TSH concentration during fasting was evident over 24 h, fasting especially decreased the absolute (1.3 ± 0.3 vs. 0.4 ± 0.1 mU/L; P < 0.02) and the relative (101 ± 18% vs. 40 ± 14%; P < 0.02) nocturnal TSH surge (mean TSH 0000-0400 h vs. mean TSH 1500-1900 h). The decreased nocturnal TSH surge during fasting was associated with a significantly decreased TSH pulse amplitude, but with an unaltered number of TSH pulses between 2000–0400 h.

In conclusion, fasting decreases 24-h TSH secretion and the nocturnal TSH surge in the absence of a change in plasma T₄ concentration. This is associated with a decreased TSH pulse amplitude, whereas TSH pulse frequency remains unchanged.

^* Part of this study was presented at the 18th Annual Meeting of the European Thyroid Association, Copenhagen, Denmark, June 1989.

Received October 30, 1989.

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