Thyroxine (T4)

Thyroxine (T4)

Thyroxine (T4), also known as tetraiodothyronine, is one of the primary hormones produced by the thyroid gland. It plays a crucial role in regulating metabolism, growth, and development in the body. T4 is synthesized from iodine and the amino acid tyrosine, and it is predominantly secreted into the bloodstream as an inactive hormone.

Production and Regulation

T4 production is stimulated by Thyroid Stimulating Hormone (TSH), which is released from the anterior pituitary gland in response to Thyrotropin-Releasing Hormone (TRH) from the hypothalamus. The synthesis of T4 involves several steps:

1. Iodine Uptake: The thyroid gland absorbs iodine from the bloodstream, which is essential for T4 production.
2. Thyroglobulin Synthesis: Iodinated tyrosines are combined within thyroglobulin, a precursor protein stored in the thyroid follicles.
3. Hormone Release: Upon stimulation by TSH, thyroglobulin is broken down to release free T4 into the bloodstream.

Approximately 80% of T4 is converted into the more active hormone triiodothyronine (T3) in peripheral tissues such as the liver and kidneys through deiodination.

Functions of T4

1. Metabolism Regulation: T4 influences how the body utilizes energy by increasing basal metabolic rate (BMR). It affects how cells metabolize carbohydrates, fats, and proteins.
2. Growth and Development: Adequate levels of T4 are essential for normal growth and development, particularly during infancy and childhood.
3. Thermoregulation: T4 contributes to maintaining body temperature by regulating metabolic processes that produce heat.
4. Cardiovascular Effects: It helps regulate heart rate and blood pressure by enhancing cardiac output.

Testing for T4 Levels

A T4 test measures the amount of thyroxine in the blood and can be performed as either a total T4 test or a free T4 test:

• Total T4 Test: Measures both bound and unbound forms of T4.
• Free T4 Test: Measures only the unbound form of T4 that is biologically active.

Normal ranges for free T4 typically fall between 0.8 to 1.8 ng/dL (nanograms per deciliter). Abnormal levels may indicate various thyroid conditions:

• High T4 Levels: Often associated with hyperthyroidism, conditions such as Graves’ disease, or excessive thyroid hormone replacement therapy.
• Low T4 Levels: Indicative of hypothyroidism, where the thyroid gland is underactive and fails to produce sufficient hormones.

Conclusion

Thyroxine (T4) is a critical hormone that regulates numerous bodily functions, particularly metabolism and growth. Monitoring T4 levels through blood tests is essential for diagnosing and managing thyroid disorders effectively.

References

1. Cleveland Clinic. (2024). Thyroxine (T4) Test: What It Is & Why It’s Done. Retrieved from Cleveland Clinic
2. Ada Health. (2023). Thyroxine T4: Functions, Levels, and Medications. Retrieved from Ada Health
3. Wikipedia. (2024). Thyroxine. Retrieved from Wikipedia
4. Healthline. (2023). What Is Thyroxine? Retrieved from Healthline