🦴 Endocrine System Lesson

PTH UP, Calcitonin DOWN: calcium homeostasis

Blood calcium is tightly controlled by three hormones working in a coordinated system — and the clinical consequences of imbalance follow a memorable pattern.

PTH

Ca2+ up

Calc.

Ca2+ down

VitD

Ca2+ up

📖 Full Breakdown

Three hormones, and the classic symptom patterns of imbalance

PTH and calcitonin have opposite effects, both feeding into the same tightly-regulated calcium level.

PTH (Parathyroid Hormone)

Released when calcium is LOW

Raises blood calcium three ways: activating osteoclasts to release calcium from bone, increasing renal reabsorption of calcium, and activating vitamin D.

Calcitonin

Released when calcium is HIGH

Produced by thyroid C cells. Lowers blood calcium by inhibiting osteoclasts — functionally the direct opposite of PTH.

Vitamin D

Increases intestinal absorption

Activated in the kidneys (from a sunlight-derived precursor). Increases how much dietary calcium is actually absorbed in the gut.

Hypocalcemia

Low calcium → tetany

Caused by hypoparathyroidism or vitamin D deficiency — presents with muscle spasms (tetany), and classic signs like Chvostek's and Trousseau's signs.

Hypercalcemia

High calcium → "stones, bones, groans, psychic moans"

Caused by hyperparathyroidism or malignancy — this classic phrase captures kidney stones, bone pain, GI complaints, and psychiatric symptoms all at once.

🩺 Clinical / Exam Application

A patient develops muscle twitching and spasms (tetany) after thyroid surgery. Because the parathyroid glands sit on the back of the thyroid and can be accidentally damaged during thyroid removal, PTH production can drop sharply post-surgery — without enough PTH to keep raising calcium, blood calcium falls, and low calcium directly causes neuromuscular excitability, producing the classic tetany. This connects directly back to the anatomical relationship covered in the Major Endocrine Glands lesson.

⚠️ Exam Alert

"Stones, bones, groans, and psychic moans" is one of the most reliably tested mnemonic phrases in endocrinology — know it cold for hypercalcemia symptom recognition (kidney stones, bone pain, abdominal groans/constipation, and psychiatric/cognitive symptoms).

🚧 Common Trap

Don't confuse which gland releases PTH versus calcitonin. PTH comes from the separate parathyroid glands; calcitonin comes from C cells within the thyroid itself — two different glands with opposite effects on the same electrolyte.

✅ Quick Check

A patient has hypoparathyroidism. Would you expect their calcium to be high or low, and what symptom would this cause?

📝 Exam Prep

Common Exam Questions

❓ What is the physiological role of PTH and what triggers its release?

✅ PTH (parathyroid hormone) is released when blood calcium is low. It raises calcium by activating osteoclasts (releasing calcium from bone), increasing renal calcium reabsorption, and activating vitamin D.

❓ What are the classic symptoms of hypercalcemia, and what phrase captures them?

✅ "Stones, bones, groans, and psychic moans" — referring to kidney stones, bone pain, GI complaints (groans), and psychiatric symptoms (psychic moans), classically caused by hyperparathyroidism or malignancy.