Which thyroid hormone is approximately 10 times more active than the other?

Thyroid hormone activity is most closely associated with the specific forms of the hormones produced by the thyroid gland. Triiodothyronine, known as T3, is the form of thyroid hormone that is approximately 10 times more active than thyroxine, or T4.

T4 is the predominant hormone secreted by the thyroid gland, but it is converted into T3 in various tissues throughout the body. This conversion is crucial because T3 has a much greater affinity for thyroid hormone receptors, which allows it to exert a more potent physiological effect. Therefore, when discussing the activity levels of these hormones, T3 takes precedence due to its greater biological potency and efficacy as a regulator of metabolic processes.

Thyroglobulin, while related to the storage and production of thyroid hormones, is not an active hormone itself; rather, it is a precursor protein that contributes to the synthesis of T4 and T3. This distinction emphasizes the significant role that T3 plays in thyroid function, as it is the active form that directly influences various metabolic activities in the body.