Although the thyroid gland evolved from the gut, there is no evidence that thyroxine functions as part of the gastro-intestinal endocrine system nor does it have any major function analogous to the control of glucose by the pancreatic islets. The control of the thyroid evolved from the pituitary control of the gonad suggesting that an early role of thyroxine was in reproduction. This idea is supported by the presence of cycles of thyroid activity associated with reproduction in both elasmobranchs and teleosts. In teleosts thyroxine is necessary for gonadal maturation. The numerous other effects of thyroxine in teleosts may have evolved from this maturational effect or have been added to it during the course of teleost evolution.