Tadpoles must undergo a major metamorphosis to become frogs. this change includes reabsorption of the tail, growth of limbs, calcification of the skeleton, increase in rhodopsin in the eye, development of lungs, change in hemoglobin structure, and reformation of the gut from the long gut of an herbivore to the short gut of a carnivore. amazingly, all of these changes are induced by thyroxine. what is the most likely explanation for such a wide array of effects of thyroxine?

There are many receptors for thyroxine on different target cells (tissues), so its effect can be huge

Explanation:

Target cell for a certain hormone is a cell that have hormone receptor specific for that hormone. There are two main types of receptors on target cell:

Hormone binding to the receptor leads to the signal cascade within the cell, which results in cell response.

Receptors for thyroid hormones are nuclear receptors.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.