Which primary thyroid hormone is most directly responsible for regulating the body’s basal metabolic rate (rate of metabolism)?

• A. Cortisol
• B. Thyroxine (T4)
• C. Triiodothyronine (T3)
• D. Calcitonin

Answer: (C)

Verified step by step guidance

1. Understand that the basal metabolic rate (BMR) is the rate at which the body uses energy while at rest to maintain vital functions such as breathing and keeping warm.
2. Recognize that thyroid hormones play a crucial role in regulating metabolism, with two main hormones produced by the thyroid gland: Thyroxine (T4) and Triiodothyronine (T3).
3. Know that Thyroxine (T4) is a prohormone and is converted into the more active form, Triiodothyronine (T3), in target tissues.
4. Identify that Triiodothyronine (T3) is the hormone that directly binds to nuclear receptors in cells to regulate gene expression and increase the basal metabolic rate.
5. Exclude other hormones listed: Cortisol is a glucocorticoid involved in stress response, and Calcitonin regulates calcium levels, so they are not primarily responsible for BMR regulation.