Thyroxine: T4
T4, also known as thyroxine, is one of the two primary hormones produced by the thyroid gland, along with triiodothyronine (T3). T4 plays a crucial role in regulating metabolism, growth, development, and various physiological functions throughout the body. Here's an overview of T4 and its functions:
1. Production and Release:
- T4 is synthesized and released by the thyroid gland in response to stimulation by thyroid-stimulating hormone (TSH), which is produced by the pituitary gland.
- T4 is secreted into the bloodstream in larger quantities compared to T3, but it is considered a prohormone because it is converted into the more active form, T3, in tissues throughout the body.
2. Metabolic Regulation:
- T4 plays a central role in regulating metabolic processes, including energy production, utilization of nutrients, and heat generation.
- It increases basal metabolic rate (BMR), which is the rate at which the body burns calories at rest, by influencing cellular metabolism and energy expenditure.
- T4 helps maintain normal body temperature by regulating heat production and dissipation.
3. Tissue Growth and Development:
- T4 is essential for normal growth, development, and maturation of tissues and organs, particularly during fetal development and childhood.
- It promotes the growth and differentiation of various tissues, including bones, muscles, and the central nervous system.
4. Cardiovascular Function:
- T4 affects cardiovascular function by influencing heart rate, cardiac output, and blood pressure.
- It helps regulate the contraction and relaxation of cardiac muscle cells and maintains cardiovascular homeostasis.
5. Neurological Function:
- T4 plays a role in the development and function of the central nervous system (CNS), including the brain.
- It is involved in neuronal migration, synaptogenesis, myelination, and neurotransmitter synthesis, contributing to cognitive function, mood regulation, and overall neurological health.
6. Regulation of Hormonal Feedback Loop:
- T4, along with T3, provides negative feedback to the hypothalamus and pituitary gland, regulating the secretion of thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) in the hypothalamic-pituitary-thyroid (HPT) axis.
- Elevated T4 levels inhibit the release of TRH and TSH, while decreased T4 levels stimulate their secretion, helping to maintain thyroid hormone homeostasis.
T4 Levels:
- The normal range of T4 levels in the blood typically falls between 4.5 and 11.2 micrograms per deciliter (μg/dL) or 58 and 145 nanomoles per liter (nmol/L), although reference ranges may vary depending on the laboratory.
- Measurement of T4 levels, along with TSH and sometimes T3 levels, is commonly used to evaluate thyroid function and screen for thyroid disorders such as hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid).
In summary, T4 is a vital hormone produced by the thyroid gland that regulates metabolism, growth, development, cardiovascular function, neurological function, and hormonal feedback mechanisms. It plays a central role in maintaining overall health and well-being throughout the body.