In this chapter, we discuss the function and pharmacological aspects of two important endocrine structures: the thyroid and parathyroid glands. Hormones secreted from the thyroid gland are involved in controlling metabolism; they also work synergistically with other hormones to promote normal growth and development. The parathyroid glands are essential in regulating calcium homeostasis and are important in maintaining proper bone mineralization.
Problems in the function of the thyroid or parathyroid glands are often treated by pharmacological methods. It is important that you understand the pharmacological management of thyroid and parathyroid function so that you can adjust your therapies appropriately for patients with bone-healing disorders and other conditions caused by problems with these endocrine glands. The normal physiological function of the thyroid gland is covered first, followed by the types of drugs used to treat hyperthyroidism and hypothyroidism. The function of the parathyroid glands is covered next, with a discussion of the role of the parathyroid glands and other hormones in maintaining bone mineral homeostasis. The final section is an overview of the drugs used to regulate bone calcification.
HORMONES OF THE THYROID GLAND
The thyroid gland lies on either side of the trachea in the anterior neck region and consists of bilateral lobes connected by a central isthmus. The entire gland weighs approximately 15 to 20 g and receives a rich vascular supply and extensive innervation from the sympathetic nervous system.1 The thyroid gland synthesizes two primary hormones: thyroxine and triiodothyronine.
Synthesis of Thyroid Hormones
The chemical structures of thyroxine and triiodothyronine are shown in Figure 31-1. Thyroid hormones are synthesized first by adding iodine to residues of the amino acid tyrosine. Addition of one iodine atom creates monoiodotyrosine, and the addition of a second iodine creates diiodotyrosine. Two of these iodinated tyrosines are then combined to complete the thyroid hormone. The combination of a monoiodotyrosine and a diiodotyrosine yields triiodothyronine, and the combination of two diiodotyrosines yields thyroxine.1,2
Structure of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). Addition of one iodine atom (I) to tyrosine produces monoiodotyrosine; addition of a second iodine atom produces diiodotyrosine. A monoiodotyrosine and diiodotyrosine combine to form triiodothyronine (T3). Coupling of two diiodotyrosines forms thyroxine (T4).
Because thyroxine contains four iodine residues, this compound is also called T4. Likewise, triiodothyronine contains three iodine residues and is called T3. There has been considerable discussion about which hormone exerts the primary physiological effects. Plasma levels of T4 are much higher than T3 levels, but T3 may exert most of the physiological effects on various tissues, which suggests that T4 is a precursor to T3 and that the ...