Triiodothyronine (T3)

Description

Triiodothyronine (T3) is one of the two primary hormones produced by the thyroid gland, a small butterfly-shaped organ located across the trachea at the base of the neck. The other thyroid hormone is called thyroxine (T4). T4 is relatively inactive and needs to be converted into T3 in the liver, kidneys, and other organs to effectively regulate the body’s metabolism.

Nearly all T3 (and T4) present in the bloodstream is bound to proteins. The remaining free (unbound) T3 is the biologically active form of the hormone, which is measured in blood tests.

High Levels

Elevated levels of free T3 are often an early sign of hyperthyroidism and can sometimes be detected before any changes in free T4 levels are observed.

Low Levels

Underproduction of thyroid hormones is associated with reduced volume in the right hippocampus (1). Low levels of free T3 are typically seen in the later stages of hypothyroidism or during severe systemic illness due to reduced conversion from T4. However, in cases of hypothyroidism, free T3 levels can remain normal for an extended period due to increased conversion from T4.