What is the role of thyroid hormones in cold-induced thermogenesis?

• A. They increase basal metabolic rate and amplify thermogenic gene expression.
• B. They increase vasodilation of cutaneous vessels.
• C. They directly activate brown fat via insulin receptors.
• D. They have no effect on thermogenesis.

Answer: (A)

Thyroid hormones raise the body's overall heat production and tune the thermogenic machinery, especially in brown fat. In cold exposure, the body activates brown adipose tissue to generate heat through UCP1, a process that converts chemical energy directly into heat. Thyroid hormones amplify this response by increasing the expression of thermogenic genes and enhancing mitochondrial capacity, so the heat produced per unit of substrate is higher. They also boost the tissue’s responsiveness to sympathetic signals that drive brown fat thermogenesis, effectively linking basal metabolic rate with the cold-induced heat you need.

That’s why the best choice is that thyroid hormones increase basal metabolic rate and amplify thermogenic gene expression. They don’t primarily cause vasodilation of skin vessels in this context, they don’t activate brown fat directly through insulin receptors, and they certainly have an effect on thermogenesis (hypothyroidism reduces it, hyperthyroidism enhances it).