Question

thyroxine and triiodothyronine work to:
a regulate calcium levels in the body.
b regulate glucose levels in the body.
c regulate sodium levels in the body.
d regulate the bodys metabolic rate.

Explanation:

Thyroxine ($T_4$) and triiodothyronine ($T_3$) are hormones produced by the thyroid gland. Their main function is to regulate the body's metabolic rate. This includes influencing how the body uses energy, affects growth and development, and impacts the function of various organs.

• Option A: Parathyroid hormone and calcitonin regulate calcium levels, not $T_3$ and $T_4$.
• Option B: Insulin and glucagon regulate glucose levels.
• Option C: Aldosterone (from the adrenal cortex) and other mechanisms regulate sodium levels.
• Option D: $T_3$ and $T_4$ increase the metabolic rate of cells, affecting processes like oxygen consumption, heat production, and macromolecule synthesis/breakdown.

Answer:

D. regulate the body's metabolic rate.