Thyroid T4 to T3 Conversion Cofactors

The Inactive Hormone

The thyroid gland primarily produces Thyroxine (T4), which is metabolically inactive. To boost metabolic rate, T4 must be converted into Triiodothyronine (T3) mainly in the liver. Many women have "Normal TSH" but suffer from hypothyroid symptoms (cold hands, hair loss, fatigue) because they are failing to convert T4 to T3.

The Deiodinase Enzymes

The conversion process is driven by Deiodinase Enzymes. These enzymes are selenium-dependent. Without adequate selenium, the body creates "Reverse T3" (rT3) instead—a "dummy" hormone that blocks the receptor without activating it.