What are the two major types of thyroid hormones?

The two major types of thyroid hormones are T3 (triiodothyronine) and T4 (thyroxine). T3 is the more active form of thyroid hormone and is responsible for regulating metabolism, heart rate, and overall growth and development. T4, while considered less active, is more abundant in circulation and serves as a precursor to T3, meaning that in the body, T4 can be converted into T3 when needed. This conversion allows for a more finely tuned regulation of metabolic processes, making T3 and T4 critical for maintaining homeostasis.

The other options listed (T1, T2, T5, and T6) do not represent recognized or significant forms of thyroid hormones. T1 and T2 do not exist as functional thyroid hormones, and T5 and T6 also do not have established roles in human physiology. Thus, the identification of T3 and T4 as the major thyroid hormones reflects our understanding of their crucial roles in endocrine function and metabolic regulation.