Journal of Clinical Endocrinology & Metabolism, Vol 67, 1309-1311, Copyright © 1988 by Endocrine Society

ARTICLES

Fasting induces a decrease in serum thyroglobulin in normal subjects

J Unger
Department of Endocrinology, School of Medicine, Campus Erasme, Free University of 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 micrograms/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 less than 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 less than 0.05) and between the mean serum Tg and T3 concentrations (r = 0.78; P less than 0.05). The individual Pearson's correlation coefficients between serum Tg and serum T3 ranged from 0.67-0.93 and were significant (r greater than 0.70; P less than 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 adaptive response to fasting.