What is the predominant form of thyroid hormone in the circulation?

What is the predominant form of thyroid hormone in the circulation?

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

Answer: (A)

Thyroxine (T4) is the predominant form circulating in the blood. The thyroid gland mainly secretes T4, which acts as a prohormone and provides a reservoir that peripheral tissues convert to the active hormone, T3, as needed. T4 has a longer half-life, so it remains in circulation at higher levels and serves as the major circulating thyroid hormone. In the blood, most of it is bound to transport proteins, with only a small free fraction available to tissues. The active hormone at target tissues, T3, is produced from T4 by deiodinase enzymes in peripheral tissues. The other iodinated tyrosines (monoiodotyrosine and diiodotyrosine) are intermediates used during hormone synthesis and are not the main circulating forms.

Thyroxine (T4) is the predominant form circulating in the blood. The thyroid gland mainly secretes T4, which acts as a prohormone and provides a reservoir that peripheral tissues convert to the active hormone, T3, as needed. T4 has a longer half-life, so it remains in circulation at higher levels and serves as the major circulating thyroid hormone. In the blood, most of it is bound to transport proteins, with only a small free fraction available to tissues. The active hormone at target tissues, T3, is produced from T4 by deiodinase enzymes in peripheral tissues. The other iodinated tyrosines (monoiodotyrosine and diiodotyrosine) are intermediates used during hormone synthesis and are not the main circulating forms.