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

Reverse T3 is a byproduct of your thyroid hormone system that doesn't activate your body's metabolism like normal thyroid hormones do. Instead, it acts like a traffic cop that redirects your thyroid hormones away from becoming active, helping your body control how much thyroid hormone is actually available to use.

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.

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.

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