Thyroid and parathyroid glands
The thyroid is a butterfly-shaped gland in the anterior neck that produces hormones regulating metabolism, while the four parathyroid glands on its posterior surface secrete parathyroid hormone to control calcium. They sit together anatomically but serve distinct endocrine functions.
The thyroid gland lies anterior to the trachea just below the larynx, with right and left lobes joined by an isthmus that typically crosses the second through fourth tracheal rings. It is the only endocrine gland that stores its hormone extracellularly, in colloid-filled follicles. Blood supply comes from the superior thyroid artery, a branch of the external carotid, and the inferior thyroid artery from the thyrocervical trunk. Two nerves make thyroid surgery hazardous: the recurrent laryngeal nerve runs near the inferior thyroid artery, and the external branch of the superior laryngeal nerve runs near the superior thyroid artery — injury causes hoarseness or a weakened voice, respectively.
Follicular cells synthesize T4 and T3 by iodinating thyroglobulin; these hormones raise basal metabolic rate, increase heat production, and support normal growth and central nervous system development. Their release is governed by the hypothalamic–pituitary axis through TRH and TSH with negative feedback. Scattered parafollicular (C) cells secrete calcitonin, which lowers serum calcium — a minor player in humans but the tumor marker for medullary thyroid carcinoma.
The parathyroid glands are four small glands, usually embedded on the posterior thyroid surface, derived embryologically from the third and fourth pharyngeal pouches. Their chief cells secrete parathyroid hormone, the primary regulator of serum calcium. PTH raises calcium by stimulating osteoclastic bone resorption, increasing renal calcium reabsorption while promoting phosphate excretion, and activating 1-alpha-hydroxylase in the kidney to produce active vitamin D, which increases intestinal calcium absorption.
Because the parathyroids sit on the thyroid, thyroidectomy can inadvertently remove or devascularize them, producing hypocalcemia with perioral tingling, tetany, and positive Chvostek and Trousseau signs. This anatomic relationship is the reason the two glands are studied together.
USMLE Step 1 covers this material in endocrine anatomy. High-yield points are the surgical nerve relationships, the pharyngeal pouch derivations, the distinction between follicular and parafollicular cells, and the mechanisms by which PTH raises serum calcium and lowers serum phosphate.
Key takeaways
- The thyroid sits anterior to the trachea in two lobes joined by an isthmus and stores hormone extracellularly as colloid.
- Follicular cells produce T4 and T3, which raise basal metabolic rate; parafollicular C cells produce calcitonin.
- The four parathyroid glands derive from the third and fourth pharyngeal pouches and lie on the posterior thyroid surface.
- PTH raises serum calcium via bone resorption, renal reabsorption, and activation of vitamin D, while promoting phosphate excretion.
- The recurrent and superior laryngeal nerves run with the thyroid arteries, and parathyroid injury during thyroidectomy causes hypocalcemia.