T3

Triiodothyronine, commonly abbreviated T3, is an active thyroid hormone essential for regulating metabolism and development. It is chemically known as 3,3',5-triiodo-L-thyronine and contains three iodine atoms. T3 is produced mainly by the thyroid gland as a result of deiodination of thyroxine (T4) in peripheral tissues, especially the liver and kidneys.

Mechanism and role: T3 binds to nuclear thyroid hormone receptors (TRα and TRβ isoforms) and modulates gene expression, influencing mitochondrial activity, transcription of metabolic enzymes, and energy expenditure. It plays a central role in controlling basal metabolic rate, heart rate and contractility, and body temperature, as well as lipid and carbohydrate metabolism. In children, T3 supports growth and brain development; in adults, it contributes to energy balance and thermogenesis.

Clinical relevance: Blood levels of T3 may be measured as free T3 (FT3) or total T3. FT3 is often used together with TSH and free T4 to assess thyroid function, particularly in suspected hyperthyroidism or pituitary disorders. In hyperthyroidism, T3 can be elevated (T3 toxicosis) even when T4 is not; in hypothyroidism, T3 may be low or normal depending on the underlying cause and duration of deficiency.

Therapeutic context: Standard therapy for hypothyroidism is levothyroxine (T4). Some patients may receive liothyronine (synthetic T3) or combination therapy, but this approach remains controversial and may carry cardiovascular risks; treatment is individualized based on clinical response and laboratory tests.