serum total thyroxine

Serum total thyroxine, often abbreviated as T4, refers to the measurement of thyroxine in the blood, including both protein-bound and free forms. Thyroxine is a crucial hormone produced by the thyroid gland, located in the lower neck. This hormone plays a vital role in regulating metabolism and growth throughout the body.123

Thyroxine is synthesized from the amino acid tyrosine and contains four iodine atoms. It is the primary hormone secreted by the thyroid gland, though it is largely inactive until converted to triiodothyronine (T3) in various tissues. Measuring serum thyroxine levels, along with thyrotropin (TSH), is a key component of thyroid function tests used to diagnose conditions like hypothyroidism, a deficiency in thyroid hormone production. Low levels of total and free thyroxine, coupled with high TSH, typically indicate hypothyroidism.342