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Fasting Decreases Thyrotropin Responsiveness to Thyrotropin-Releasing Hormone: A Potential Cause of Misinterpretation of Thyroid Function Tests in the Critically III^*

Divisions of Endocrinology and Metabolism, Departments of Medicine and Clinical Investigation, National Naval Medical Center, Walter Reed Army Medical Center Washington DC 20307
the Uniformed Services University of the Health Sciences Bethesda, Maryland 20014

Address requests for reprints to: Dr. Kenneth D. Burman, Department of Clinical Investigation, Endocrine-Metabolic Service, 7D, Walter Reed Army Medical Center, Washington, D. C. 20307.

We have previously reported that caloric deprivation inhibits peripheral T₄ metabolism and blunts the TSH response to TRH in euthyroid obese subjects. To determine whether these phenomena also occur in hypothyroid subjects, T₄, T₃, rT₃, and the TSH response to TRH were measured initially and after a 60-h fast in sevenhypothyroid patients. Short term fasting caused a 29% decrement in the maximum serum TSH increment and a 32% decrement in the integrated TSH response to TRH (P < 0.01). In two subjects with mild hypothyroidism, basal TSH as well as the TSH response to TRH were reduced to levels within the normal range. Specifically, basal TSH values decreased from 7.6 to 3.5 µU/ml and from 11 to 4.1 µU/ml. In the seven subjects, mean serum T₃ decreased significantlyfrom 88 to 60 ng/dl, (P < 0.05) and rT₃, initially undetectable in six of seven subjects, rose to detectable or low normal values in four of sevensubjects, serum T₄ remained at 2.7 µg/dl during both study periods.

We conclude that 1) fasting induces changes in both peripheral thyroid hormone metabolism and the hypothalamic-pituitary axis in hypothyroid individuals which are qualitatively similar to those that occur in euthyroid subjects; and 2) in certain hypothyroid subjects, fasting alone can decrease basal TSH values to within the normal range. If these data can be extrapolatedto critically ill subjects whose caloric intake may be diminished, they suggest that basal TSHconcentrations in moderately and severely hypothyroid critically ill subjects will accurately reflect the biochemically hypothyroid state. However, mild degrees of hypothyroidism in critically ill subjects might be overlooked due to the lowering effect of fasting or poor caloric intake alone on basal TSH concentrations.

^* This work was supported in part by CICC 0-06-1288 from the Bureau of Medicine and Surgery, Navy Department, Washington, D. C. The opinions and assertions expressed are the private ones of the authors and are not to be construed as official or reflecting the view of the Departments of the Navy or Army, the Naval Service at large, or the Department of Defense.

Received January 5, 1983.

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