Question

Why thyroxine is a hormone not an enzyme?

• A. It is secreted in small quantity.
• B. It is not a polypeptide.
• C. It has no special effect.
• D. It is directly poured into blood.

Answer 1

The correct option is D It is directly poured into blood.

An enzyme is a highly selective catalyst, that greatly accelerates both the rate and specificity of metabolic reactions.

A hormone, on the other hand, is a chemical released by a cell or a gland in one part of the body, that sends out messages, that affect cells in other parts of the organism.

The thyroid hormones, triiodothyronine ($T_3$) and its prohormone, thyroxine ($T_4$), are tyrosine-based hormones produced by the thyroid gland, that are primarily responsible for regulation of metabolism. Hence, thyroxine is a hormone.