T4, also known as thyroxine, is a hormone produced by the thyroid gland in the human body. It is an essential component of the thyroid hormone system and plays a crucial role in regulating metabolism, growth, and development. T4 is considered a prohormone, as it undergoes conversion into the more active form of thyroid hormone called triiodothyronine (T3) in various tissues.

T4 is produced and released into the bloodstream by the thyroid gland in response to stimulation by thyroid-stimulating hormone (TSH) released from the pituitary gland. It is transported through the bloodstream, where it binds to proteins for circulation and delivery to target cells. Once inside the cells, T4 is converted to T3 by the removal of an iodine atom, which is the metabolically active form of thyroid hormone.

The levels of T4 in the body are tightly regulated to maintain normal physiological functions. Abnormal levels of T4 can indicate an underlying thyroid disorder, such as hypothyroidism (low T4 levels) or hyperthyroidism (excess T4 levels). T4 levels are commonly measured through blood tests to assess thyroid function and diagnose thyroid-related conditions.

T4 replacement therapy is often prescribed to individuals with hypothyroidism to supplement the insufficient hormone levels and restore normal metabolic activity. Additionally, T4 medications can be used in the management of certain types of thyroid cancer. Such medications generally come in synthetic forms, such as levothyroxine, and are designed to mimic the natural T4 hormone.