Chapter 8: Endocrine System

The endocrine system operates through circulating hormones that bind to target cell receptors to regulate fundamental physiological processes including growth, metabolism, and fluid homeostasis. Dysregulation typically arises from tumors, inflammatory processes, or autoimmune mechanisms that result in either excessive hormone secretion or deficiency, diagnosed through measurement of circulating hormone levels, assessment of free active hormones, and specialized stimulation or suppression testing protocols. The hypothalamus and anterior pituitary form a critical regulatory axis wherein hypothalamic-releasing factors control secretion of growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, and gonadotropins, while the posterior pituitary directly releases antidiuretic hormone and oxytocin from hypothalamic neurons. The thyroid gland produces metabolic hormones T3 and T4, with hyperthyroidism most commonly resulting from Graves disease, an autoimmune condition causing hypermetabolic symptoms, while hypothyroidism typically stems from Hashimoto thyroiditis leading to metabolic suppression and developmental complications in untreated neonates. The parathyroid glands maintain calcium homeostasis through parathyroid hormone secretion, with hyperparathyroidism causing bone resorption and hypercalcemia, whereas adrenal pathology manifests as Cushing syndrome from excess cortisol, Addison disease from glucocorticoid deficiency, or congenital adrenal hyperplasia from enzymatic defects redirecting steroid synthesis toward androgen production. The endocrine pancreas maintains glucose homeostasis through insulin and glucagon secretion, with type 1 diabetes resulting from autoimmune beta cell destruction and type 2 involving progressive insulin resistance and beta cell dysfunction. Additionally, multiple endocrine disorders can occur simultaneously through autoimmune polyglandular syndromes, inherited multiple endocrine neoplasia tumor syndromes, and paraneoplastic syndromes where nonendocrine tumors ectopically produce hormones, creating complex clinical presentations requiring integrated diagnostic and therapeutic approaches.