Endocrine Glands Anatomy: Pituitary, Thyroid, Parathyroid, and Adrenal Glands for Med Students

The endocrine system includes many organs that produce hormones (the pancreas, gonads, hypothalamus, and even the heart and kidneys), but the four discrete ANATOMICAL glands that med students study in anatomy blocks are the pituitary, thyroid, parathyroid, and adrenal glands. Each has a specific location, blood supply, venous drainage, innervation, and hormone profile. Pituitary gland: sits in the sella turcica of the sphenoid bone, connected to the hypothalamus by the infundibulum (pituitary stalk). Two parts: the anterior pituitary (adenohypophysis) releases 6 hormones under hypothalamic control (GH, prolactin, ACTH, TSH, FSH, LH), and the posterior pituitary (neurohypophysis) stores and releases oxytocin and ADH produced in the hypothalamus. Blood supply: superior and inferior hypophyseal arteries from the internal carotid. Thyroid gland: located in the anterior neck, spanning C5 to T1 levels, wrapping around the trachea. Two lobes connected by an isthmus overlying the 2nd-4th tracheal rings. Blood supply: superior thyroid artery (from external carotid) and inferior thyroid artery (from thyrocervical trunk). Produces T3, T4 (from follicular cells) and calcitonin (from parafollicular C cells). The thyroid has the highest blood flow per gram of any organ in the body. Parathyroid glands: typically 4 small glands on the posterior surface of the thyroid (2 superior, 2 inferior). Number varies — 10-15% of people have 3, 5, or 6. Parafollicular to the thyroid, they regulate calcium via parathyroid hormone (PTH). The clinical importance: surgeons doing thyroidectomies have to be extremely careful not to damage the parathyroids or their blood supply, because accidental parathyroidectomy causes hypocalcemia and tetany. Adrenal glands: paired triangular glands sitting on top of each kidney. Each has two functionally distinct parts: the cortex (outer) with three zones producing three different types of steroid hormones, and the medulla (inner) derived from neural crest cells producing catecholamines (epinephrine, norepinephrine). The cortex and medulla are developmentally distinct — the cortex is mesodermal, the medulla is neuroectodermal (essentially a modified sympathetic ganglion). Snap a photo of any endocrine gland diagram or question and AnatomyIQ identifies the gland, traces the blood supply, lists the hormones, and explains clinical correlations including the high-yield USMLE topics. This content is for educational purposes only and does not constitute medical advice.

The pituitary gland sits in a bony depression in the sphenoid bone called the sella turcica (Turkish saddle), immediately below the optic chiasm and the hypothalamus. It is approximately the size of a pea (about 1 cm in diameter) and weighs less than a gram — yet controls most of the endocrine system through its hormone output. The infundibulum (pituitary stalk) connects the gland to the hypothalamus above, transmitting both neural signals and portal blood flow. Two embryologically and functionally distinct parts: the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis). **Anterior pituitary**: derived from Rathke's pouch (an ectodermal evagination from the roof of the embryonic oral cavity). It is a TRUE endocrine gland with its own hormone-producing cells organized by cell type. The 6 anterior pituitary hormones: 1. Growth Hormone (GH) — stimulates growth, bone elongation, protein synthesis. 2. Prolactin (PRL) — stimulates milk production in lactating women. 3. Adrenocorticotropic Hormone (ACTH) — stimulates adrenal cortex to release cortisol. 4. Thyroid Stimulating Hormone (TSH) — stimulates thyroid to release T3/T4. 5. Follicle Stimulating Hormone (FSH) — gonadal function. 6. Luteinizing Hormone (LH) — gonadal function. Anterior pituitary is controlled by hypothalamic hormones delivered via the hypophyseal portal system — a specialized capillary network that allows hypothalamic releasing hormones (GHRH, TRH, CRH, GnRH) and inhibiting hormones (somatostatin, dopamine) to reach the anterior pituitary cells directly without entering systemic circulation. **Posterior pituitary**: derived from a downgrowth of neural ectoderm from the floor of the diencephalon. It is NOT a true endocrine gland — it is an extension of the hypothalamus. The hormones (ADH and oxytocin) are PRODUCED in hypothalamic neurons (supraoptic and paraventricular nuclei) and transported down axons through the infundibulum to be STORED and RELEASED from the posterior pituitary terminals. The posterior pituitary has no hormone-producing cells of its own. **Clinical correlations**: - Pituitary adenomas: benign tumors of the anterior pituitary. The most common functional adenoma is prolactinoma (which causes galactorrhea and amenorrhea in women, hypogonadism in men). Non-functional adenomas cause mass effects — headaches and bitemporal hemianopsia from compression of the optic chiasm sitting directly above the gland. - Sheehan syndrome: postpartum pituitary infarction from severe blood loss during delivery. The pituitary is engorged during pregnancy and particularly susceptible to ischemia. Classic presentation: failure to lactate (low prolactin), amenorrhea, adrenal insufficiency over the following months. - Empty sella syndrome: CSF herniates into the sella turcica, compressing the pituitary. Usually asymptomatic. - Diabetes insipidus: ADH deficiency. Central DI (pituitary/hypothalamic) vs nephrogenic DI (kidney unresponsive to ADH). Presentation: massive dilute urine output and thirst. AnatomyIQ generates pituitary anatomy questions with clinical scenarios testing the common USMLE Step 1 patterns.

The thyroid gland is a butterfly-shaped structure in the anterior neck, wrapping around the trachea from approximately C5 to T1. Two lateral lobes connect via a midline isthmus that typically overlies the 2nd to 4th tracheal rings. Some individuals have a pyramidal lobe — a small extension from the isthmus up toward the hyoid bone (a remnant of the thyroglossal duct from embryonic development). **Blood supply of the thyroid**: extremely rich, two arteries per side: 1. Superior thyroid artery — branch of the external carotid artery. Approaches the gland from above. 2. Inferior thyroid artery — branch of the thyrocervical trunk (which comes off the subclavian artery). Approaches from below. 3. In about 10% of people, an additional thyroid ima artery arises from the aortic arch or brachiocephalic trunk. This rich vascular supply is why thyroid surgery requires meticulous hemostasis and why hyperthyroid states (like Graves disease) produce an audible thyroid bruit. **Venous drainage**: superior, middle, and inferior thyroid veins. Superior and middle drain into the internal jugular vein. Inferior drains into the brachiocephalic vein. **Thyroid cells and hormones**: - Follicular cells: produce T3 and T4 (thyroid hormones). Require iodine for synthesis. Make up the vast majority of the gland. - Parafollicular C cells: produce calcitonin (regulates calcium by inhibiting bone resorption). Scattered between follicles. **Parathyroid glands**: typically 4 small pea-sized glands on the posterior surface of the thyroid. Two superior parathyroids are usually located at the level of the cricoid cartilage; two inferior parathyroids at the level of the inferior thyroid pole. Number variation: 10-15% of people have 3 or 5 parathyroids; rare cases have 6. Ectopic parathyroids can be found in the mediastinum (from failed descent during embryology) or within the thyroid parenchyma itself. **Parathyroid hormone (PTH)**: regulates blood calcium. When calcium is low, PTH is released and acts on bone (increases resorption, releasing calcium), kidney (increases calcium reabsorption, increases vitamin D activation), and indirectly on gut (via vitamin D, increases calcium absorption). Net effect: raises blood calcium. **High-yield clinical correlations**: - Thyroidectomy complications: damage to the recurrent laryngeal nerve (RLN) which runs in the tracheoesophageal groove deep to the thyroid. Unilateral RLN injury causes hoarseness. Bilateral injury causes airway compromise. Damage to the external branch of the superior laryngeal nerve (runs with the superior thyroid artery) causes loss of high-pitched voice. - Hypoparathyroidism after thyroidectomy: the parathyroids are small and their blood supply is fragile. Even with careful surgery, accidental devascularization of the parathyroids leads to hypocalcemia that can cause tetany, Trousseau sign (carpal spasm with BP cuff inflation), and Chvostek sign (facial twitch with tapping over facial nerve). - Goiter: enlargement of the thyroid from various causes (iodine deficiency, Graves disease, Hashimoto thyroiditis). A large goiter can compress the trachea and esophagus. - Thyroid cancer: papillary thyroid cancer is the most common. Associated with neck radiation history. - Primary hyperparathyroidism: usually caused by a solitary parathyroid adenoma. Presentation: 'stones, bones, groans, and psychiatric overtones' (kidney stones, bone pain, abdominal pain, depression). Classic USMLE topic. AnatomyIQ walks through thyroid and parathyroid anatomy with surgical correlations — particularly the RLN and vascular anatomy that thoracic and head-and-neck surgeons need to know.

The adrenal glands (also called suprarenal glands) are paired triangular structures sitting on top of each kidney in the retroperitoneum. They are approximately 4-6 cm in their longest dimension and weigh about 4-5 grams each. Each adrenal gland has two functionally and developmentally distinct parts that happen to be in the same anatomical location: the cortex (outer) and the medulla (inner). **Adrenal cortex**: derived from mesoderm. Produces steroid hormones from cholesterol. The cortex has three histological and functional zones, from outside to inside: 1. Zona glomerulosa (outermost): produces mineralocorticoids, primarily aldosterone. Aldosterone regulates sodium and potassium balance by acting on the distal tubule of the kidney to reabsorb sodium (and water) and excrete potassium. Stimulated by angiotensin II (via the renin-angiotensin system) and by high plasma potassium. 2. Zona fasciculata (middle): produces glucocorticoids, primarily cortisol. Cortisol has widespread effects: raises blood glucose (gluconeogenesis), suppresses the immune system, affects mood and cognition, influences fat distribution. Controlled by ACTH from the anterior pituitary. 3. Zona reticularis (innermost): produces sex steroids, primarily androgens (DHEA, DHEA-S, androstenedione). Small amounts in both sexes. Clinically important in adrenal tumors that overproduce androgens, causing virilization in women. Mnemonic: 'GFR — Salt, Sugar, Sex' or 'The deeper you go, the sweeter it gets' (glomerulosa-fasciculata-reticularis, with salt/sugar/sex steroid products). **Adrenal medulla**: derived from neural crest cells (same lineage as sympathetic nervous system neurons). The medulla is essentially a modified sympathetic ganglion without postganglionic axons — instead, the chromaffin cells release catecholamines (epinephrine 80%, norepinephrine 20%) directly into the bloodstream upon sympathetic stimulation. This is why the medulla is sometimes called the 'endocrine arm of the sympathetic nervous system.' **Blood supply** (rich and complex): - Superior suprarenal artery (from inferior phrenic artery) - Middle suprarenal artery (direct branch from abdominal aorta) - Inferior suprarenal artery (from renal artery) Multiple small arteries converge to supply the cortex, which then drains through a central medullary vein. This unique pattern means the medulla is exposed to very high concentrations of cortisol from the cortex before the blood leaves the gland — which is important because cortisol is required for the enzyme PNMT in medullary chromaffin cells that converts norepinephrine to epinephrine. No cortex → no epinephrine production. **Venous drainage** (high-yield anatomy tested on exams): - Right adrenal vein drains directly into the INFERIOR VENA CAVA (very short vein). - Left adrenal vein drains into the LEFT RENAL VEIN, which then drains into the IVC. This asymmetry is clinically important because the short right adrenal vein can tear easily during surgical removal (right adrenalectomy), and the left adrenal vein's drainage through the renal vein means left-sided tumors can invade the renal vein. **Clinical correlations**: - Addison disease: primary adrenal insufficiency. Autoimmune destruction of the adrenal cortex. Presents with fatigue, weight loss, hyperpigmentation (from elevated ACTH causing melanocyte stimulation), hypotension, hyponatremia, hyperkalemia. - Cushing syndrome: hypercortisolism. Various causes including pituitary ACTH-secreting tumors (Cushing disease), adrenal cortical tumors, ectopic ACTH production (lung cancer). Presents with moon facies, buffalo hump, central obesity, striae, muscle weakness, diabetes. - Conn syndrome: primary hyperaldosteronism, usually from aldosterone-secreting adrenal adenoma. Presents with hypertension, hypokalemia, metabolic alkalosis. - Pheochromocytoma: catecholamine-secreting tumor of the adrenal medulla. Classic triad of episodic hypertension, palpitations, and diaphoresis. Rule of 10s: 10% bilateral, 10% extra-adrenal, 10% familial, 10% malignant, 10% calcified, 10% kids. - Congenital adrenal hyperplasia: enzyme deficiencies (most commonly 21-hydroxylase deficiency) causing altered steroid production with compensatory adrenal hyperplasia. AnatomyIQ generates USMLE-style questions on adrenal anatomy and clinical correlations — particularly the high-yield venous drainage asymmetry and the pheochromocytoma rule of 10s.