What is the predominant form of thyroid hormone in circulation?

• A. Thyroxine
• B. Triiodothyronine
• C. Diiodotyrosine
• D. Monoiodotyrosine

Answer: (A)

Thyroxine, also known as T4, is the predominant form of thyroid hormone found in circulation. It is produced primarily by the thyroid gland and plays a crucial role in regulating metabolism and growth. Although Triiodothyronine (T3) is the more biologically active form of thyroid hormone and is involved in numerous bodily functions, it is present in smaller amounts in the bloodstream compared to T4.

Thyroxine is converted into T3 in peripheral tissues, which underscores its significance as a precursor hormone. In addition, T4 has a longer half-life than T3, which allows it to be more stable and readily available in the circulation for conversion as needed by the body.

Diiodotyrosine and Monoiodotyrosine are intermediates in the synthesis of thyroid hormones but are not found in significant amounts in circulation. Therefore, the correct identification of thyroxine as the predominant form reflects its major role in the body's hormonal balance and metabolic processes.