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Fasting Induces a Decrease in Serum Thyroglobulin in Normal Subjects^*

Departments of Endocrinology and Nuclear Medicine and Institute of Interdisciplinary Research, School of Medicine, Campus Erasme, Free University of Brussels Brussels, Belgium

Address requests for reprints to: Dr. Jacques Unger, IRIBHN, Campus Erasme, Route de Lennik 808, 1070 Brussels, Belgium.

Fasting decreases serum T₃ concentrations by reducing peripheral conversion of T₄ to T₃. Whether it affects TSH and thyroid secretion is controversial. Thyroglobulin (Tg) is a secretory product of the thyroid gland. To determine if fasting might alter thyroid secretion, we measured serum Tg concentrations during and after a 4-day fast in seven normal subjects and examined its correlation with serum TSH, T₄, and T₃ concentrations.

Serum TSH concentrations significantly decreased from a mean prefasting value of 1.55 ± 0.19 (±SE) mU/L to a nadir of 0.82 ± 0.09 mU/L (P = 0.01) after 3 days of fasting. The mean serum Tg level decreased from 30 ± 5 to 12 ± 3 µg/L (P = 0.005) and serum T₃ from 160 ± 8 (2.46 ± 0.12) to 69 ± 7 ng/dL (1.06 ± 0.11 nmol/L) (P < 0.0001) after 4 days of fasting. Serum T₄ did not change significantly. Serum TSH, T₃, and Tg concentrations returned to prefasting values within 3 days after initiation of refeeding. A significant correlation was found between the mean serum TSH and Tg concentrations (r = 0.77; P < 0.05) and between the mean serum Tg and T₃ concentrations (r = 0.78; P < 0.05). The individual Pearson’s correlation coefficients between serum Tg and serum T₃ ranged from 0.67–0.93 and were significant (r > 0.70; P < 0.05) in five of the seven subjects.

In conclusion, the parallel changes in serum TSH, Tg, and T₃ concentrations during fasting suggest that in addition to the well known decrease in serum T₃ concentrations, decreased TSH and thyroid secretion (reflected by serum Tg) may be an adapative response to fasting.

^* This work was supported by a grant from the Ministère de la Politique Scientifique (Actions Concertées) and the Fonds de la Recherche Scientifique Medicale (FRSM 3.4539.84).

Received April 21, 1988.