Primary Hyperparathyroidism from Parathyroid Adenoma, Not Thyroid Pathology
This patient has primary hyperparathyroidism (PHPT) from a parathyroid adenoma, not a thyroid adenoma—the thyroid mass is likely an incidental finding or represents an intrathyroidal parathyroid adenoma. The combination of hypercalcemia (ionized calcium 1.38 mmol/L, elevated above normal 1.15-1.29), hypophosphatemia (1.5 mg/dL), and inappropriately normal PTH (28 pg/mL) in the setting of hypercalcemia is diagnostic of PHPT 1, 2.
Why This Is Parathyroid Disease, Not Thyroid Disease
The PTH level is "inappropriately normal" for the degree of hypercalcemia—in a healthy person, elevated calcium should suppress PTH to undetectable levels (<20 pg/mL), but this patient's PTH remains in the normal range, indicating autonomous parathyroid hormone secretion 1, 2.
The biochemical triad of elevated ionized calcium, low phosphorus, and non-suppressed PTH is pathognomonic for primary hyperparathyroidism, not thyroid pathology 1, 2.
Thyroid adenomas do not cause this metabolic profile—they may cause hyperthyroidism but do not directly affect calcium, phosphorus, or PTH levels 2.
The Thyroid Mass: Two Possibilities
Most Likely: Intrathyroidal Parathyroid Adenoma
Parathyroid adenomas can be located within the thyroid gland itself (intrathyroidal location), appearing as a "thyroid mass" on imaging but actually representing ectopic parathyroid tissue 3, 4.
Giant intrathyroidal parathyroid adenomas have been reported causing hypercalcemic crisis with altered mental status, mimicking thyroid pathology on physical examination 3.
These lesions require both parathyroidectomy and potentially partial thyroidectomy for complete excision 3.
Alternative: Coincidental Thyroid Nodule
The thyroid mass may be an incidental thyroid nodule unrelated to the hypercalcemia, with a separate parathyroid adenoma located elsewhere in the neck 1.
Thyroid nodules are extremely common (present in 50-70% of the general population on ultrasound) and their presence does not exclude parathyroid disease 1.
Critical Concern: Altered Mental Status from Hypercalcemic Crisis
The altered mental status is directly caused by the hypercalcemia (ionized calcium 1.38 mmol/L = approximately 5.5 mg/dL, which is elevated) and represents a medical emergency 2, 5.
Hypercalcemia causes neurological symptoms including confusion, somnolence, and altered consciousness when severe or rapidly developing 2.
This patient requires immediate treatment with IV hydration and bisphosphonates (zoledronic acid or pamidronate) to lower calcium levels acutely 2.
Diagnostic Workup Required Immediately
Confirm PHPT and Localize the Adenoma
Obtain 25-hydroxyvitamin D level to exclude vitamin D deficiency as a cause of secondary hyperparathyroidism (though the elevated ionized calcium makes this unlikely) 1.
Order parathyroid imaging with ultrasound and/or 99mTc-sestamibi SPECT/CT to localize the adenoma for surgical planning—this will determine if the "thyroid mass" is actually an intrathyroidal parathyroid adenoma 1, 3.
Measure 24-hour urinary calcium to assess for hypercalciuria and evaluate kidney involvement 1.
Assess renal function with serum creatinine and eGFR, as hypercalcemia can cause acute kidney injury 1.
Rule Out Malignancy-Associated Hypercalcemia
Although the non-suppressed PTH makes PHPT most likely, measure PTHrP (parathyroid hormone-related peptide) if there is any concern for occult malignancy, as this would be elevated in malignancy-associated hypercalcemia with suppressed PTH 1.
The normal PTH essentially excludes malignancy-associated hypercalcemia, which typically presents with suppressed PTH (<20 pg/mL) and elevated PTHrP 1, 2.
Immediate Management Algorithm
Acute Treatment of Hypercalcemic Crisis
Aggressive IV hydration with normal saline (200-300 mL/hour initially) to restore intravascular volume and promote calciuresis 2.
Administer IV bisphosphonate (zoledronic acid 4 mg over 15 minutes or pamidronate 60-90 mg over 2-4 hours) after volume repletion 2.
Monitor serum calcium every 6-12 hours until stable and improving 1.
Avoid loop diuretics until after volume restoration, as they can worsen dehydration 2.
Definitive Treatment: Surgery
Urgent referral to an experienced parathyroid surgeon for parathyroidectomy is indicated given the hypercalcemic crisis and altered mental status 1, 3.
Surgical indications are clearly met: corrected calcium >1 mg/dL above upper limit of normal, symptomatic disease (altered mental status), and likely impaired kidney function 1.
If imaging confirms intrathyroidal location, the surgeon should be prepared for combined parathyroidectomy and partial thyroidectomy 3.
Critical Pitfall: Hungry Bone Syndrome Post-Operatively
After removal of a large, long-standing parathyroid adenoma, severe postoperative hypocalcemia (hungry bone syndrome) is highly likely, requiring aggressive calcium and vitamin D replacement 3.
Prepare for postoperative calcium supplementation with high-dose IV calcium gluconate and oral alfacalcidol/calcitriol, with frequent calcium monitoring 3.
Hungry bone syndrome can cause severe symptomatic hypocalcemia (calcium as low as 1.65 mmol/L) requiring weeks to months of intensive replacement therapy 3.
What This Is NOT
This is NOT thyroid disease—thyroid adenomas do not cause hypercalcemia, hypophosphatemia, or elevated PTH 2.
This is NOT secondary hyperparathyroidism—secondary hyperparathyroidism presents with hypocalcemia or normal calcium, not hypercalcemia 1.
This is NOT vitamin D toxicity—vitamin D toxicity would suppress PTH to undetectable levels 1, 2.