The T3 test quantitatively measures triiodothyronine (T3) levels in the blood to assess thyroid function.

T3 is one of the primary hormones secreted by the thyroid gland and plays a key role in regulating the body’s metabolic processes. Thyroid activity is regulated by thyroid-stimulating hormone (TSH), which is secreted by the pituitary gland and stimulates the thyroid to produce T3 and T4.