thyroksyna

Thyroksyna, also known as thyroxine (T4), is a major thyroid hormone produced by the thyroid gland. It exists in two main forms in humans, the prohormone T4 and the more active metabolite T3 (triiodothyronine); most T3 is formed by peripheral conversion of T4 in tissues. Synthesis occurs in the thyroid follicles through iodination of tyrosine residues on thyroglobulin and subsequent coupling reactions. The resulting hormones are stored in the colloid as part of thyroglobulin and released into the bloodstream in response to thyroid-stimulating hormone (TSH) from the pituitary.

In circulation, thyroid hormones are bound to carrier proteins, primarily thyroxine-binding globulin (TBG), with smaller fractions bound to albumin and transthyretin. Only a small free fraction is biologically active. T4 has a relatively long plasma half-life, about seven days, and is constantly converted to the active T3 in various tissues by deiodinase enzymes. Reverse T3 (rT3) is an inactive metabolite formed when deiodination favors alternative pathways.

Thyroksyna regulates many aspects of metabolism and development. It increases basal metabolic rate, enhances carbohydrate and lipid breakdown, promotes protein synthesis, and supports growth and nervous system development, especially during early life. In adults, it influences heart rate, thermogenesis, energy levels, and mood.

Clinically, thyroksyna is central to thyroid disease management. Hypothyroidism is commonly treated with levothyroxine, a synthetic form of T4, aiming to restore normal TSH and free T4 levels. Hyperthyroidism treatment may involve reducing hormone production or blocking its effects. Diagnosis typically relies on serum TSH with free T4 (and sometimes free T3) measurements. Dosing and monitoring must consider age, cardiac status, pregnancy, and potential interactions that affect absorption or metabolism. Excess thyroksyna can cause tachycardia, anxiety, weight loss, and bone loss with long-term use.