How are FT3 and Stress related?

The relationship between FT3 (free triiodothyronine) and stress is mediated primarily by the hypothalamic-pituitary-adrenal (HPA) axis. Chronic stress elevates cortisol, which can inhibit the conversion of T4 to the active FT3, leading to a condition known as 'low T3 syndrome' or 'euthyroid sick syndrome.' This results in lower FT3 levels without underlying thyroid disease. Conversely, acute stress may transiently increase thyroid hormone release. Evidence is largely mechanistic and clinical, but individual responses vary. Importantly, a single FT3 measurement is insufficient to assess stress; it must be interpreted alongside TSH, FT4, cortisol, and symptoms. MyBody-X does not directly measure FT3; such blood tests require medical supervision. If stress-related thyroid changes are suspected, consult an endocrinologist.