Fasting blunts the TSH response to synthetic thyrotropin-releasing hormone (TRH)

Thyrotropin (TSH) responses to intravenous thyrotropin-releasing hormone (TRH) were studied in 9 men after 12 and 36-h fasts separated by more than a week and performed in random order. The TSH, basally and in response to TRH, was significantly lower after the 36-h fast compared to that after 12 h. The mechanism for this effect is not clear, but may be related to the altered hormonal or fuel status associated with prolonged fasting.