Zoology 10th Edition by Stephen Miller,John Harley

Exercise 6

In tadpoles in the prematamorphic stage, the hypothalamus stimulates the adenohypophysis to secrete TSH. TSH then stimulates the thyroid gland to secrete thyroxine. Thyroxine binds to its receptor and initiates the change in gene expression necessary for metamorphosis. As metamorphosis proceeds, thyroxine reaches its maximal level, after which the forelimbs begin to form and the tail is reabsorbed. Melatonin has broad effects in various animals, such as synchronizing activity patterns with light intensity and day lengths and controlling variations in skin color.
Explain the evolutionary origin of the thyroid gland and its functions in vertebrates.