The Correct Answer is C
Primary hyperparathyroidism is characterized by elevated or inappropriately normal PTH levels, NOT low PTH—making option C the exception and the correct answer to this question. 1
Why Each Statement is True or False:
A) Most Common Cause of Hypercalcemia - TRUE
Primary hyperparathyroidism is indeed the most frequent cause of hypercalcemia in ambulatory patients 2, 3. In hospitalized patients, malignancy becomes the leading cause, but in the outpatient setting, PHPT dominates.
B) More Common in Older Women - TRUE
The condition is most common in postmenopausal women 3, 4. While it can occur at any age, including in pregnant women, the typical demographic is older females.
C) Characterized by Low PTH - FALSE (This is the Exception)
This is the incorrect statement. The biological diagnosis of primary hyperparathyroidism is based on a parathyroid hormone value that is inappropriate to the blood calcium value 1. The typical form combines:
- Hypercalcemia
- Elevated PTH
- Increased calciuria or calcium excretion fraction
Even in atypical presentations, PTH is either frankly elevated or inappropriately normal (not suppressed) in the context of hypercalcemia 1. When hypercalcemia occurs with suppressed/low PTH, this suggests alternative diagnoses such as malignancy, hypervitaminosis D, or other non-parathyroid causes 5.
D) Parathyroid Adenoma is Most Common Cause - TRUE
The most common lesion in PHPT is the solitary benign parathyroid adenoma 6. Parathyroid carcinomas are uncommon, and multiglandular disease occurs in approximately 15% of cases, often associated with familial syndromes 6.
E) May Be Familial in Small Percentage - TRUE
In approximately 15% of patients, all four parathyroid glands are involved, which may be associated with familial hereditary syndromes such as:
- Multiple endocrine neoplasia types 1, 2A, and 4
- PHPT jaw tumor syndrome 6
Genetic testing is suggested in patients with PHPT below age 30 or at any age with multigland disease 6.
Key Clinical Pitfall:
The most important diagnostic error to avoid is dismissing PHPT when PTH levels appear "normal" on laboratory reference ranges. In the context of hypercalcemia, a PTH level in the normal range is physiologically inappropriate—the parathyroid glands should be suppressed. Even PTH values as low as 50 pg/mL (within normal range of 12-88 pg/mL) can represent PHPT when calcium is elevated, with cure rates of 96.7% after parathyroidectomy 7.