The fT3/fT4 ratio reveals how efficiently the body converts thyroxine (T4, the storage hormone) into triiodothyronine (T3, the metabolically active hormone).
The fT3/fT4 ratio reveals how efficiently the body converts thyroxine (T4, the storage hormone) into triiodothyronine (T3, the metabolically active hormone). A low ratio in the context of normal or elevated T4 indicates poor peripheral conversion — often missed when only TSH is measured.
Low (poor conversion): selenium deficiency, chronic illness, caloric restriction, elevated cortisol, liver disease, high-dose beta-blockers. T4-only thyroid medication can paradoxically lower the ratio. High: hyperthyroidism, thyroiditis releasing stored T3, T3-producing nodules, excess T3 supplementation.
Low ratio: persistent fatigue despite normal TSH, unexplained weight gain, cold intolerance, constipation, dry skin, hair loss, brain fog, depression — the classic hypothyroid cluster with "normal" standard thyroid tests.
Selenium-rich foods: Brazil nuts (1–2/day provides full RDI), tuna, sardines, eggs. Zinc from pumpkin seeds, meat, legumes. Adequate iodine from seaweed or iodised salt.
Soy and raw cruciferous vegetables in iodine-deficient states. Extreme caloric restriction suppresses T3 production.
Educational information only — not a diagnosis. Every Pheno result is interpreted in context and reviewed by a physician.
See all 119 markers we test →