Thyroid hormones are largely made up of which element?

Thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3), are predominantly composed of iodine. Iodine is an essential element for the synthesis of these hormones, as it is incorporated into their molecular structure. The thyroid gland takes up iodine from the bloodstream, and this iodine is then used to create the active forms of the hormones. Insufficient iodine intake can lead to disorders such as hypothyroidism and goiter, highlighting the critical role that iodine plays in thyroid function and hormone production. This is why the correct answer to the question is iodine, as it directly relates to the makeup of thyroid hormones and their crucial physiological roles.