Thyroxine: T4

T4, also known as thyroxine, is one of the two main thyroid hormones produced by the thyroid gland in the neck. The other hormone is T3 (triiodothyronine). T4 is synthesized by the thyroid gland and released into the bloodstream, where it is transported throughout the body to influence various physiological functions.

T4 is produced and released by the thyroid gland in response to the signals from the hypothalamus and pituitary gland, which are part of the feedback loop that regulates thyroid hormone production. T4 is considered the prohormone or precursor to T3, as it can be converted into T3 in the peripheral tissues of the body, such as the liver, kidneys, and skeletal muscle.

The primary function of T4 is to regulate metabolism and energy levels in the body. It influences the rate at which cells use oxygen and produce heat, as well as the rate at which nutrients are metabolized. T4 is also important for the growth and development of the body, particularly the brain and nervous system, during fetal and early childhood stages.

T4 levels in the blood are regulated by the hypothalamic-pituitary-thyroid axis. The hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to produce thyroid-stimulating hormone (TSH). TSH then signals the thyroid gland to produce and release T4 and T3.

Abnormal T4 levels can indicate various thyroid disorders, such as hypothyroidism (low T4 levels) or hyperthyroidism (high T4 levels). Hypothyroidism can cause symptoms such as fatigue, weight gain, and cold intolerance, while hyperthyroidism can cause symptoms such as weight loss, sweating, and anxiety.

T4 levels can be measured through a blood test, along with other thyroid hormone levels and thyroid function tests. Treatment for thyroid disorders may involve hormone replacement therapy, which may include T4 supplementation in the form of levothyroxine. Treatment is typically managed by an endocrinologist or healthcare provider specializing in thyroid disorders.