How calcium is tested on Step 1

Start with what each hormone does to calcium and phosphate.

PTH↑ Ca²⁺, ↓ phosphate; bone resorption, renal Ca²⁺reabsorption, activates vitamin DCalcitriol↑ Ca²⁺ and ↑ phosphate absorption from the gutCalcitoninFrom thyroid C cells; ↓ bone resorption (minor role)FGF23From bone; ↓ phosphate reabsorption and ↓ calcitriol
Calcium regulation. Only ionized calcium is active; low albumin lowers total but not ionized calcium.

Parathyroid lab patterns

Match the three values to the disorder.

DisorderCalciumPhosphatePTH
Primary hyperparathyroidism↑↓↑
Secondary hyperparathyroidism (CKD)↓ or normal↑↑
Tertiary hyperparathyroidism↑↑↑↑
Hypercalcemia of malignancy (PTHrP)↑↓↓
Vitamin D excess, granulomatous disease↑↑↓
Hypoparathyroidism↓↑↓
Pseudohypoparathyroidism↓↑↑
Vitamin D deficiency↓↓↑
Familial hypocalciuric hypercalcemiaMildly ↑NormalNormal or mildly ↑

Familial hypocalciuric hypercalcemia (calcium-sensing receptor defect) has low urine calcium and needs no surgery.

High and low calcium

Symptoms reflect nerve and muscle excitability.

  • Hypercalcemia: kidney stones, bone pain and osteitis fibrosa cystica, abdominal pain and constipation, polyuria (nephrogenic DI), confusion, short QT.
  • Hypocalcemia: numbness around the mouth, muscle cramps, Chvostek sign (facial twitch), Trousseau sign (carpal spasm with a blood pressure cuff), seizures, prolonged QT.
  • Causes of hypoparathyroidism: thyroid or parathyroid surgery, autoimmune disease, DiGeorge syndrome (22q11 deletion), low magnesium.
  • Pseudohypoparathyroidism type 1a: end-organ resistance to PTH (GNAS) with Albright hereditary osteodystrophy: short stature and short fourth and fifth metacarpals.

Multiple endocrine neoplasia

Inherited autosomal dominant syndromes of several endocrine tumors.

MEN syndromesMEN1 (menin)Parathyroid, pituitary,pancreasMEN2A (RET)Medullary thyroid, pheo,parathyroidMEN2B (RET)Medullary thyroid, pheo,neuromas
MEN syndromes. RET carriers get a preventive thyroidectomy; medullary thyroid cancer makes calcitonin and shows amyloid stroma.

Worked example

Link the timing to the cause.

functions Worked example β€” tingling after thyroid surgery

The day after a total thyroidectomy, a patient has tingling around the mouth and carpal spasm when a blood pressure cuff is inflated. What are the expected calcium, phosphate, and PTH levels, and the treatment?

  1. Perioral tingling and Trousseau sign suggest hypocalcemia.
  2. Thyroid surgery can remove or damage the parathyroid glands.
  3. Without PTH, calcium falls and phosphate rises because the kidney stops excreting it.
  4. Symptomatic hypocalcemia is treated with IV calcium gluconate, then oral calcium and calcitriol.

Answer. Low calcium, high phosphate, low PTH (hypoparathyroidism); give IV calcium gluconate.