During a prolonged fast of 48 hours, the hypothalamic-pituitary-thyroid axis undergoes an adaptive change. Which statement BEST describes this adaptation?

• A TRH secretion increases to compensate for reduced T3 levels from decreased peripheral conversion
• B Reduced TRH secretion and increased rT3 production lower metabolic rate in a non-thyroidal illness pattern ✓
• C TSH secretion increases to stimulate thyroid hormone synthesis despite adequate iodine availability
• D Increased T4-to-T3 conversion in peripheral tissues maintains euthyroid state

Explanation

During prolonged fasting, the body downregulates the HPT axis as an energy-conserving adaptation. Peripheral deiodination of T4 shifts toward producing reverse T3 (rT3, physiologically inactive) rather than active T3. Hypothalamic TRH secretion is also reduced. This pattern (low T3, high rT3, normal or low TSH) is termed 'sick euthyroid syndrome' or 'non-thyroidal illness syndrome' and reduces BMR. Options A, C, and D describe responses that would inappropriately maintain or increase metabolic rate during energy deficit.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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