What is a thyroid monitoring test?

Triiodothyronine (T3) and thyroxine (T4) are the two main hormones produced by the thyroid gland. T3 is the more powerful of the two hormones, which regulate metabolism throughout the body. T4 acts both as a hormone by itself and as a precursor for T3 production. The vast majority of T3 and T4 in the body is bound to other proteins, primarily thyroid-binding globulin (TBG).

The remaining unbound T3 and T4 are called free T3 and T4. To stimulate the production of T3 and T4, the pituitary gland secretes thyroid-stimulating hormone (TSH). Reverse T3 (rT3) is a metabolite of T4 that is biologically inactive but functions as a competitive T3 inhibitor.

It is normal for there to be some rT3 in the blood but elevated levels of rT3 have been shown in intensive care patients and in some severe disorders. Typical thyroid testing will include T3, T4 and thyroid-stimulating hormone. Though little research has been performed on rT3, its inclusion in this test can offer a broader picture of thyroid health.

Why take the test?

This test measures the level of T3 (free and bound), T4 and thyroid-stimulating hormone in the blood. It also includes an additional test for reverse T3. The test is typically used to diagnose and monitor thyroid problems.