Thyroid hormone physiology is widely misunderstood — even by physicians who treat thyroid disease daily. The thyroid gland produces both T4 and T3, but predominantly T4 (approximately 93% T4, 7% T3 directly from the thyroid). T4 is largely biologically inert — it is a prohormone, a storage form, that must be converted to the biologically active T3 before it can bind to thyroid hormone receptors and exert metabolic effects.
The Conversion Pathway
T4-to-T3 conversion is catalyzed by deiodinase enzymes (D1, D2, D3), primarily in the liver (approximately 60% of conversion) and gut (approximately 20%), with the remaining conversion occurring in peripheral tissues including the brain, muscle, and kidneys. The efficiency of this conversion varies significantly between individuals and is affected by multiple factors:
Nutritional status: Selenium is the essential cofactor for all three deiodinase enzymes. Selenium deficiency directly impairs T4-to-T3 conversion. Iron deficiency also impairs conversion. Both are common in the populations most likely to have thyroid symptoms — women of reproductive age and older adults.
Chronic stress and cortisol: Elevated cortisol directly inhibits D1 deiodinase activity, impairing T4-to-T3 conversion and shunting T4 toward Reverse T3 production. This is one mechanism by which chronic stress produces functional hypothyroidism even in patients with intact thyroid glands and normal TSH.
Inflammation: Systemic inflammation, cytokine elevation (IL-6, TNF-α), and oxidative stress all reduce deiodinase activity and impair conversion. Patients with autoimmune conditions, obesity, or chronic infections often have poor conversion independent of thyroid gland function.
The Reverse T3 Complication
T4 can be converted either to active T3 (via D1/D2 deiodinase) or to inactive Reverse T3 (via D3 deiodinase). Reverse T3 is structurally similar to T3 and binds to T3 receptors — but does not activate them, effectively blocking T3 from binding. In states of physiological stress (illness, starvation, psychological stress, inflammation), the body preferentially shunts T4 toward Reverse T3 as a protective metabolic downshift.
The Free T3:Reverse T3 ratio (optimal above 0.2) provides a practical clinical marker of thyroid hormone bioavailability at the cellular level. A patient with a ratio below 0.1 has significant T3 receptor blockade regardless of their TSH or Free T4 values.
T3 Supplementation: The Combination Therapy Evidence
The landmark study by Celi et al. (JCEM, 2011) demonstrated that patients switched from T4 monotherapy to combination T3/T4 therapy showed significantly greater improvements in quality of life, mood, body weight, and cognitive function. The DIO2 polymorphism — a genetic variant affecting D2 deiodinase function — identifies a specific population of patients who are poor T4-to-T3 converters and who consistently show better outcomes on combination therapy than on T4 alone. Testing for this variant is available and clinically actionable.