All vertebrates have a thyroid gland. In mammals, it is usually bilobed and located just caudal to the larynx, adjacent to the lateral surface of the trachea. The two lobes may be connected by a fibrous isthmus (eg, ruminants, horses), or a connecting isthmus may be indistinct (eg, dogs, cats). The gland is extremely vascular. In birds, it is found within the thoracic cavity; both lobes are located near the syrinx, adjacent to the carotid artery near the origin of the vertebral artery.
Ectopic or accessory thyroid tissue is relatively common in most species, especially dogs and cats. It may be located anywhere from the larynx to the diaphragm and may be responsible for maintaining normal thyroid function after surgical thyroidectomy. In addition, ectopic thyroid tissue occasionally is the site of hyperplasia or neoplasia.
Thyroid hormones are the only iodinated organic compounds in the body. Thyroxine (T4) is the main secretory product of the normal thyroid gland. However, the gland also secretes 3,5,3′-triiodothyronine (T3), reverse T3, and other deiodinated metabolites. T3 is ~3–5 times more potent than T4, whereas reverse T3 is thyromimetically inactive.
Although all T4 is secreted by the thyroid, a considerable amount of T3 is derived from T4; therefore, T4 has been called a prohormone. Its activation to the more potent T3 is a step regulated individually by peripheral tissues.
Thyroid hormone secretion is regulated primarily via negative-feedback control through the coordinated response of the hypothalamic-pituitary-thyroid axis: thyrotropin-releasing hormone (TRH) binds to the thyrotroph cell in the pituitary and stimulates secretion of thyrotropin (thyroid-stimulating hormone, TSH), which binds to the follicular cell membrane and stimulates thyroid hormone synthesis and secretion.
Thyroid hormones are water-insoluble lipophilic compounds that are bound to plasma proteins (thyroxine-binding protein, thyroxine-binding prealbumin [transthyretin], and albumin). The major function of the thyroid hormone–binding proteins is probably to provide a hormone reservoir in the plasma and to “buffer” hormone delivery into tissue. In healthy euthyroid animals, 0.1% of total serum T4 is free (not bound to thyroid hormone–binding proteins), whereas ~1% of circulating T3 is free. Evidence suggests that the fractions of circulating free T4 and free T3 determine the amount of hormone available for uptake by tissues.
Thyroid hormones act on many different cellular processes; however, no single reaction or metabolic event can be equated with their action. Although both T4 and T3 have intrinsic metabolic activity, T3 is 3–5 times more potent in binding to the nuclear receptors and similarly more potent in stimulating oxygen consumption.
Effects of thyroid hormones generally are divided into two categories: those that manifest within minutes to hours after hormone receptor binding and do not require protein synthesis, and those that manifest later (usually >6 hours) and require synthesis of new proteins. Approximately half the increase in oxygen consumption produced by thyroid hormones is related to activation of the plasma membrane–bound Na+/K+ ATPase; thyroid hormones also stimulate mitochondrial oxygen consumption. These changes are linked directly to the calorigenic effect of thyroid hormones. More chronic effects invariably are related to the cellular actions that require interaction with nuclear T3 receptors, followed by an increase in protein synthesis crucial to physiologic processes such as growth, differentiation, proliferation, and maturation.
Thyroid hormones, in physiologic quantities, are anabolic. In conjunction with growth hormone and insulin, protein synthesis is stimulated and nitrogen excretion is reduced. However, in excess (hyperthyroidism), they can be catabolic, with increased gluconeogenesis, protein breakdown, and nitrogen wasting.