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 eU/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 dur...
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