How do hormones control amphibian metamorphosis?

Amphibian metamorphosis is a complex, coordinated process primarily controlled by the endocrine system, which relies on a hormonal signal sent through the blood to tissues throughout the body.1

The process is initiated by the hypophysis, which secretes thyrotropic hormone. This hormone activates the thyroid gland to produce and release thyroxine and other iodine-containing compounds. These thyroid hormones are the primary drivers of metamorphosis; they circulate in the blood and trigger diverse morphological and biochemical changes, such as the resorption of the tadpole's tail and the development of limbs. The specific effect of these hormones depends on the "competence" of the target tissues, meaning different cells respond to the same hormone in different ways—for instance, stimulating limb growth while simultaneously causing tail muscle degeneration.123

Additionally, prolactin plays a regulatory role in amphibians, acting as a larval growth hormone that can inhibit the transition from larva to adult. Metamorphosis thus depends on a delicate balance between these hormonal outputs and the changing sensitivities of the body's tissues.43