Chapter 28 Diseases of the Endocrine System
The endocrine glands and hormones in birds are similar to those found in mammals (Box 28-1). The glands release hormones into the circulation. These hormones activate a target organ to produce changes within the body. As in mammals, there is a hypothalamohypophysial axis composed of the hypothalamus and the pituitary gland. Releasing hormones are produced in the hypothalamus that are carried by the portal circulation to the anterior pituitary, where they stimulate the release of the appropriate stimulating hormone into the general circulation. Stimulating hormones released from the anterior pituitary (the adenohypophysis) include follicle-stimulating hormone, luteinizing hormone, thyroid-stimulating hormone, prolactin, growth hormone, and adrenocorticotropic hormone. The posterior pituitary (the neurohypophysis) releases arginine vasotocin (AVT) and mesotocin (MT).
Box 28-1 ENDOCRINE ORGANS OF THE AVIAN SPECIES
Both AVT and MT are produced in the hypothalamus and stored in the posterior pituitary. The effect of AVT is similar to antidiuretic hormone in mammals. It acts to control water reabsorption and oviductal contraction during oviposition. AVT also decreases glomerular filtration, which causes increased water retention. The role of MT currently is unknown.
Disease of the Thyroid Gland
The thyroid glands are located just cranial to the thoracic inlet and lateral to the trachea. They are enervated by both the parasympathetic and sympathetic systems. As in mammals, the glands are composed of follicles that produce both triiodothyronine (T3) and tetraiodothyronine (T4). The thyroid glands of birds do not contain C cells for the production of calcitonin. In the bird, calcitonin is produced by C cells found in the ultimobranchial bodies that are just caudal to the parathyroid glands. The function of calcitonin is unknown and does not appear to reduce serum levels of calcium as it does in mammals. Iodine is concentrated in the gland, but the thyroid hormones contain a greater percentage of iodine than those of mammals. The most common disease associated with the thyroid gland is related to iodine deficiency, resulting in formation of a goiter. This disease is more commonly associated with parakeets, budgies, canaries, pigeons, and cockatiels than the larger parrots.