Decreased Serum Thyrotropin Induced by Fasting

Abstract

The objective of this study was to investigate the effect of fasting on serum thyrotropin (TSH). Serum triiodothyronine (T₃), thyroxine (T₄), TSH and prolactin (PRL) were measured twice daily by radioimmunoassay, and thyroidal iodine release (TIR) was measured continuously in two normal and six obese euthyroid subjects before, during and after a 7-day fast. All subjects displayed inhibition of TIR which fell to a nadir within 3 days, followedby a gradual rise with continued fasting. A further prompt rise in TIR in 5 of the 8 subjects began within 6 h of refeeding with a 1,000 calorie, oral carbohydrate load. Serum TSH (mean ± SE) also fell significantly within 2 days (P < 0.005) from 2.4 ± 1.0 to 1.1 ± 1.0 εU/ml, then gradually rose to near baseline values, followed by an increase to 2.9 ± 1.3 μU/ml after refeeding. The pattern of TSH response paralleled the observed changes in TIR, indicating dependenceof TIR on circulating TSH during fasting and refeeding. Serum T3 fell from 117 ± 10 to 54 ± 7 ng/dl during fasting (P < 0.001), and progressively roseto 99 ± 9 ng/dl, 48 h after refeeding. There was no significant change in total serum T₄ concentration or free thyroxine index throughout the study. Possible hormonal mechanisms for these observations were also studied. Mean serum PRL concentrations showed no significant change in the eight euthyroid subjects. There was nochange in the daily excretion of 17-hydroxycorticosteroids or urine cortisol, or in the diurnal pattern of serum cortisol with fasting and refeeding. Metyrapone administration did not reverse the inhibitory effects of fasting on TIR or TSH in two normal subjects. The present study confirms the finding of other investigators that fasting lowers serum T₃ values to about 50% of baseline. At the same time a transient inhibition of serum TSH was observed. The inhibition of TSH secretion, concurrent with reduced peripheral conversion of T₄ to T₃ may be of physiological adaptive value in response to the fasting state.