Thyroid hormone

Thyroid hormone refers primarily to the endogenous hormones thyroxine (T4) and triiodothyronine (T3), which are produced by the thyroid gland and regulate metabolic rate, energy expenditure, thermogenesis, and numerous aspects of growth and development. Pharmaceutical thyroid hormone products are synthetic or animal-derived versions of these hormones (most commonly levothyroxine as synthetic T4, less commonly liothyronine as synthetic T3, or desiccated thyroid extract containing both T4 and T3). In medical practice, these are used as prescription drugs to replace or suppress endogenous thyroid hormone production. In the supplement marketplace, some over-the-counter products marketed for thyroid support have been found to contain undeclared T3 and T4 in pharmacologically relevant doses, effectively acting as unregulated thyroid hormone therapy. These preparations can acutely alter thyroid function in otherwise healthy individuals, potentially leading to symptoms of hyperthyroidism such as palpitations, anxiety, weight loss, and heat intolerance. Unlike targeted nutraceuticals that support thyroid physiology indirectly (such as iodine, selenium, or myo-inositol), products that provide actual thyroid hormones directly override normal hypothalamic–pituitary–thyroid regulation and function more like drugs than dietary supplements. Because of this, thyroid hormone is not considered a general wellness supplement for healthy people. Its appropriate role is as a prescription therapy for clearly diagnosed thyroid disorders under medical supervision. Use by euthyroid, otherwise healthy individuals without a documented thyroid hormone deficit is not supported by evidence and carries real risks, especially when sourced from unregulated “thyroid support” supplements that may be misbranded or adulterated.