Triiodothyronine

Total triiodothyronine (T3) measures the full amount of T3 in your blood, including both protein-bound and free hormone. T3 is the biologically active thyroid hormone and is far more potent than T4 at the receptor level. Most of the T3 in your body is produced outside the thyroid gland itself, by the conversion of T4 into T3 in peripheral tissues (notably the liver, kidneys, and skeletal muscle). Because only the unbound (free) T3 fraction can enter your cells and drive metabolic activity, and because total T3 is strongly affected by binding-protein levels, free T3 (FT3) is usually the preferred test for assessing thyroid hormone availability.

Total T3 still has a role, particularly when investigating T3 thyrotoxicosis (a form of hyperthyroidism where T3 is disproportionately elevated), monitoring response to T3-containing thyroid medications, or interpreting results where binding-protein abnormalities are suspected. Think of total T3 as the cargo manifest for the active thyroid hormone, it counts every T3 molecule in circulation, whether in transit or ready to deliver, while free T3 counts only the molecules currently available to work.

Your total T3 level responds to the same binding-protein influences as total T4: pregnancy, HRT, and the contraceptive pill tend to raise it; androgens, severe illness, and nephrotic syndrome tend to lower it. Conversion of T4 to T3 also matters, illness, stress, very low calorie diets, and deficiencies in selenium and zinc can reduce T3 even when T4 is adequate. If your total T3 is outside the expected range, your GP will interpret it alongside TSH, free T4, and free T3 to understand what is going on.