T3: The Metabolically Active Thyroid Hormone
T3 binds to nuclear thyroid hormone receptors in virtually every cell in the body, directly regulating gene expression involved in metabolic rate, thermogenesis, cardiac output, bone metabolism, and neurological function. Because most T3 is produced peripherally from T4 conversion, the thyroid system has two potential failure points: thyroid production (detected by TSH and T4) and peripheral conversion (detected only by measuring T3 directly). Many patients with normal TSH and T4 but low T3 experience classic hypothyroid symptoms because their cells are not receiving adequate active hormone.
The Missing Test
Most doctors do not order Free T3, yet it is the hormone actually doing the work in your cells. Low T3 causes symptoms even when TSH and T4 look normal.
Nutrient-Dependent Conversion
Converting T4 to T3 requires selenium, zinc, and iron. Deficiencies in these nutrients are extremely common and can impair conversion.
Optimal Free T3 Benchmarks
Common Questions
Why is Free T3 not routinely tested?
Standard medical practice relies on TSH as a screening tool and adds Free T4 if TSH is abnormal. Free T3 is considered unnecessary by many guidelines, but functional medicine practitioners consider it essential.
Can stress lower Free T3?
Yes. Chronic stress increases cortisol, which inhibits the deiodinase enzymes that convert T4 to T3 and promotes conversion of T4 to Reverse T3 (inactive form) instead.