Chapter 45: Thyroid & Parathyroid Disorders – Hormone Therapy

The thyroid, the body's largest endocrine gland, produces thyroxine (T4) and triiodothyronine (T3), hormones essential for fetal development, basal metabolic rate, and cardiac function, with T3 being the more potent, biologically active form. Regulation occurs via the hypothalamic-pituitary-thyroid (HPT) axis, which employs a negative feedback system involving TRH and TSH to maintain adequate circulating thyroid hormone levels. Diagnosing dysfunction relies heavily on measuring serum TSH, the most sensitive marker, complemented by free T4 and T3 levels, and antibody testing to identify autoimmune etiologies like Hashimoto's thyroiditis (hypothyroidism) or Graves’ disease (hyperthyroidism). Hypothyroidism, characterized by a deficiency in thyroid hormones, is typically treated with synthetic levothyroxine (LT4), which is the standard of care due to its long half-life and the ability of peripheral tissues to convert T4 to T3 as needed. Optimal LT4 absorption requires careful administration on an empty stomach and separation from various interacting medications and foods containing metals like calcium or iron. Conversely, hyperthyroidism or thyrotoxicosis involves excessive thyroid hormone exposure, often due to Graves’ disease. Treatment choices—which must be individualized—include antithyroid drugs (ATDs) like methimazole (MMI) or propylthiouracil (PTU), radioactive iodine (RAI) ablation, or surgery (thyroidectomy). ATDs inhibit hormone synthesis and offer a chance for remission but carry risks of serious adverse effects such as hepatotoxicity (higher risk with PTU) and agranulocytosis. Adjunctive agents, notably nonselective beta-blockers, are used to rapidly control bothersome adrenergic symptoms like palpitations and tremor while awaiting definitive therapy. Finally, the chapter addresses parathyroid disorders, focusing on the regulation of calcium by parathyroid hormone (PTH). Primary hyperparathyroidism, often caused by an adenoma, results in hypercalcemia, with surgery being the only curative treatment. For non-surgical candidates, the calcimimetic cinacalcet is used to increase calcium receptor sensitivity on the parathyroid gland, thus lowering PTH and serum calcium.