Which thyroid hormone is described as the predominant hormone produced by the thyroid in the source material?

• A. Calcitonin
• B. None of these
• C. T3
• D. T4

Answer: (C)

Thyroid hormone output is primarily about which hormone the gland releases in the larger amount and how its activity relates to what tissues do with it. The thyroid secretes mainly thyroxine (T4). It’s the prohormone, and most circulating T3—the more active form—comes from conversion of T4 in peripheral tissues like the liver and kidneys. Calcitonin is produced by C cells and has a separate role in calcium homeostasis, not in regulating metabolism in the same way as T4 and T3.

If a source states that T3 is the predominant hormone produced by the thyroid, that wording may be pointing to its greater potency as the active hormone, rather than the amount released by the gland. The standard fact to remember is that the gland secretes mostly T4, with T3 largely formed afterward from T4.