What are the Parathyroid Glands?
The parathyroid glands are four tiny, pea-sized structures embedded on the posterior surface of the thyroid gland. Despite their small size, they play an outsized role in regulating blood calcium levels through parathyroid hormone (PTH).
The parathyroid glands secrete PTH, which raises blood calcium by stimulating bone resorption, increasing renal calcium reabsorption, and activating vitamin D to boost intestinal calcium absorption.
- 1.Low blood Ca2+ — Calcium-sensing receptors on parathyroid cells detect a drop
- 2.PTH secretion — Parathyroid glands release parathyroid hormone (PTH)
- 3.Bone resorption — PTH stimulates osteoclasts to release calcium from bone
- 4.Renal reabsorption — Kidneys reabsorb more calcium and excrete more phosphate
- 5.Vitamin D activation — PTH activates vitamin D, boosting intestinal calcium absorption
- 6.Ca2+ normalizes — Rising blood calcium suppresses further PTH release
Step-by-step worked examples
There are typically four parathyroid glands, each about the size of a grain of rice. What is their approximate individual weight?
Each gland weighs approximately 30–40 mg Combined weight of all four ≈ 120–160 mg Tiny size makes them easy to miss during thyroid surgery
Normal serum calcium is 8.5–10.5 mg/dL. If a patient's PTH-driven bone resorption raises calcium from 7.8 mg/dL by 1.4 mg/dL, what is the new level, and is it in range?
7.8 + 1.4 = 9.2 mg/dL 9.2 mg/dL falls within 8.5–10.5 mg/dL PTH successfully restored normal calcium levels
PTH increases renal calcium reabsorption while promoting phosphate excretion. If a patient excretes 900 mg phosphate/day at baseline and PTH increases excretion by 25%, what is the new daily phosphate excretion?
900 × 0.25 = 225 mg increase 900 + 225 = 1125 mg/day PTH's phosphaturic effect explains the rise
Flashcards
Quick quiz
Q1.What is the main function of the parathyroid glands?
Q2.What triggers PTH secretion?
Q3.How does PTH affect bone?
Q4.How many parathyroid glands does a person typically have?
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Common mistakes
The parathyroid glands are part of the thyroid's hormone-producing tissue. — Correct: They are separate glands, embedded in but functionally distinct from the thyroid.
PTH lowers blood calcium. — Correct: PTH raises blood calcium; calcitonin (from the thyroid) lowers it.
PTH has no effect on phosphate. — Correct: PTH increases renal phosphate excretion, lowering blood phosphate levels.
Everyone has exactly four parathyroid glands in fixed positions. — Correct: Most people have four, but the number and exact location can vary anatomically.
FAQ
What are the parathyroid glands?
Four small glands on the back of the thyroid that secrete PTH to regulate blood calcium levels.
What is the parathyroid hormone pathway/formula?
Low Ca2+ → PTH release → bone resorption + renal reabsorption + vitamin D activation → Ca2+ rises.
What are examples of PTH action in the body?
Restoring calcium after a low-calcium meal, or maintaining bone calcium balance over years, are both driven by PTH.
How are parathyroid gland disorders detected?
Blood tests for PTH and calcium levels, along with imaging, are used to diagnose hyper- or hypoparathyroidism.