Review Question - QID 108781

The most likely diagnosis in this patient is primary hyperparathyroidism due to a parathyroid adenoma. In primary hyperparathyroidism, parathyroid hormone (PTH) is increased, calcium is increased, and phosphate is decreased.

The presence of bony aches, constipation, and kidney stones is suggestive of symptomatic hypercalcemia, which is characterized by “bones, groans (constipation), stones, and psychiatric overtones.” Furthermore, the presence of a mass in the inferior pole of the thyroid suggests that this patient's metabolic abnormalities may stem from a parathyroid adenoma secreting excess PTH. Parathyroid hormone functions to increase serum calcium and decrease serum phosphate in response to hypocalcemia or hypomagnesemia. This is accomplished by increasing bone resorption, increasing kidney resorption of calcium, decreasing kidney resorption of phosphate, and increasing production of 1,25-hydroxyvitamin D. In cases of primary hyperparathyroidism, PTH secretion is abnormally high, leading to elevated serum calcium and decreased serum phosphate.

Incorrect Answers:
Answer 2: The combination of decreased calcium, increased phosphate, and increased PTH is consistent with secondary hyperparathyroidism. In this condition, parathyroid hyperplasia occurs in response to other factors causing hypocalcemia or hyperphosphatemia (e.g., chronic kidney disease and vitamin D deficiency).

Answer 3: The combination of increased calcium, decreased phosphate, and decreased PTH is consistent with an etiology of hypercalcemia that is independent of PTH such as milk alkali syndrome. The low PTH is due to a normal downregulation response to elevated calcium.

Answer 4: The combination of decreased calcium, increased phosphate, and decreased PTH may arise in the setting of hypoparathyroidism. Without PTH, the kidney will have decreased calcium resorption and increased phosphate resorption. There will also be a decreased bone resorption of calcium and phosphate and decreased production of calcitriol due to decreased stimulation of 1-alpha-hydroxylase in the kidney.

Answer 5: This patient has signs and symptoms consistent with primary hyperparathyroidism. Serum metabolites and PTH levels would not be normal in this patient.

Bullet Summary:
Parathyroid adenomas are the most common cause of primary hyperparathyroidism. In this condition, parathyroid hormone will be abnormally elevated, leading to high serum calcium and low serum phosphate.