Conditions That Mimic Hyperparathyroidism

Hyperparathyroidism is a common, insidious but treatable disease. These are conditions that MIMIC hyperparathyroidism, and are managed without parathyroidectomy.

Conditions That Mimic Hyperparathyroidism

Hypercalcemia of Malignancy

• This is the most common cause of hypercalcemia in hospitalized patients

• Tumors (lung, breast, renal, squamous cell) secrete PTHrP (PTH-related peptide), which activates PTH receptors — labs show high calcium, low PTH, elevated PTHrP

Familial Hypocalciuric Hypercalcemia (FHH)

• Inactivating mutation in the calcium-sensing receptor (CaSR)

• Mimics primary hyperparathyroidism almost exactly: high calcium, inappropriately normal or elevated PTH

• Key distinguishing feature: very low urine calcium (Ca/Cr clearance ratio <0.01)

• A family history of unsuccessful parathyroidectomy is another clue

Renal Leak (Renal Calcium Leak)

• In renal leak, the kidneys let too much calcium escape into the urine.

• The parathyroid glands respond by making more parathyroid hormone (PTH) to keep the blood calcium normal.

• Because PTH is high and urine calcium is high, often with kidney stones, renal leak can look like primary hyperparathyroidism on early lab tests.

• The key difference is that blood calcium is usually normal in renal leak, while it is usually high in primary hyperparathyroidism.

• Renal leak is generally ruled out before normocalcemic primary hyperparathyroidism is diagnosed.

Granulomatous Diseases

• Sarcoidosis, tuberculosis, histoplasmosis, berylliosis

• Activated macrophages produce 1,25-(OH)₂ vitamin D autonomously → hypercalcemia

• PTH is suppressed (not elevated)

Vitamin D Toxicity

• Excess exogenous vitamin D → hypercalcemia with suppressed PTH

Immobilization Hypercalcemia

• Especially in young patients or those with high bone turnover; PTH is low

Hyperthyroidism

• Increased bone resorption can cause mild hypercalcemia; PTH suppressed

Adrenal Insufficiency

• Can cause hypercalcemia through uncertain mechanisms; PTH usually suppressed

Paget's Disease (with immobilization)

• Rarely causes hypercalcemia unless combined with immobility

Drugs That Mimic Hyperparathyroidism

Drug/Substance Mechanism

1. Thiazide diuretics (have “-thiazide” in the name) and thiazide-like diuretics (Chlorthalidone, Indapamide, Metolazone) Decrease renal calcium excretion → hypercalcemia; can unmask primary hyperparathyroidism. In a case of hypercalcemia with an unsuppressed PTH in a patient on any thiazide or thiazide-like agent, holding the drug for 4–6 weeks and rechecking labs before pursuing a hyperparathyroidism workup may be useful. If calcium normalizes, the drug was the cause. If it persists, primary hyperparathyroidism moves back to the top of the list.

2. Lithium Shifts the calcium-sensing receptor (CaSR) set point, raises PTH secretion threshold — can cause true PTH elevation

3. Calcium supplements (excess) Direct calcium load; suppresses PTH

4. Vitamin D / analogues Increases GI absorption; suppresses PTH

5. Vitamin A / retinoids Increase bone resorption

6. Milk-alkali syndrome (calcium + absorbable antacids: calcium carbonate, sodium bicarbonate, or calcium citrate) Hypercalcemia + alkalosis; PTH suppressed

7. Theophylline toxicity Rare; can cause hypercalcemia

Key Lab Clue

The most important distinguishing test is PTH level:

• Elevated or inappropriately normal PTH + high calcium → primary hyperparathyroidism, FHH, or lithium effect

• Suppressed PTH + high calcium → malignancy (check PTHrP), granulomatous disease, vitamin D toxicity, or drug effect

Urine calcium and PTHrP help sort out the rest.