parathyroid gland

Meaning: The parathyroid glands are small endocrine glands, usually four, on the back of the thyroid. Their cells sense low plasma Ca²⁺ and release parathyroid hormone (PTH), a peptide hormone that works through a cell-surface receptor and a second messenger. PTH raises blood Ca²⁺ by stimulating osteoclast-mediated bone resorption (through osteoblast signals), increasing Ca²⁺ reabsorption in the kidney, and promoting kidney activation of vitamin D, which increases intestinal Ca²⁺ absorption. Rising Ca²⁺ then suppresses PTH release, a negative feedback loop.

On the MCAT: PTH is paired with calcitonin from the thyroid, which lowers blood Ca²⁺ by favoring bone deposition. Expect to predict outcomes. Removing the parathyroids lowers blood Ca²⁺, and low Ca²⁺ makes nerves and muscle more excitable, causing spasms. An overactive gland raises blood Ca²⁺ and weakens bone. Passages may also test peptide hormone signaling and feedback loops.

Common confusion: PTH raises blood Ca²⁺ but lowers blood phosphate, because it increases phosphate excretion in urine even though bone breakdown releases both. Also, the parathyroids are not controlled by the pituitary. Blood Ca²⁺ acts on them directly. They are separate from the thyroid despite sharing its location, and they make a different hormone.