The thyroid hormone T4 is changed into a more active form called T3 in organs like the liver and kidneys through a biochemical process.

Thyroxine (T4) must be converted into triiodothyronine (T3) to become biologically active, and this conversion requires two specific enzymes called deiodinase type 1 and type 2. Changes in the genes that produce these enzymes can reduce the body's ability to activate thyroid hormone.

The thyroid hormone T4 can be converted into reverse T3, a molecule that binds to thyroid hormone receptors without activating them, which reduces the overall effect of active thyroid hormone on cells.

The body converts the thyroid hormone T4 into its active form, T3, using enzymes that require selenium as a component.

Reverse T3 is a naturally occurring thyroid hormone byproduct that makes up a significant portion of thyroid hormones in your blood. It's created when your body converts another hormone called T4, and its levels stay steady enough that doctors can easily measure them with current lab tests.

Your body uses iodine to build thyroid hormones — without iodine, it can't make the key hormones that help control your metabolism.