Hashimoto's Thyroiditis and Laboratory Abnormalities
The elevated calcium and polycythemia in this patient are NOT directly caused by Hashimoto's thyroiditis or hypothyroidism; these findings require separate investigation for primary hyperparathyroidism and secondary polycythemia, respectively.
Understanding the Laboratory Findings
Hypercalcemia (Calcium 10.4 mg/dL)
This mild hypercalcemia with elevated TSH suggests two separate conditions rather than a causal relationship. While hypothyroidism has historically been listed as a potential cause of hypercalcemia, this association lacks robust evidence and appears to be based on inadequate exclusion of primary hyperparathyroidism in older case reports 1.
Primary hyperparathyroidism (PHPT) is the most likely explanation for the hypercalcemia, accounting for approximately 90% of hypercalcemia cases in the general population 2.
The co-occurrence of Hashimoto's thyroiditis and PHPT is well-documented, with studies showing a 1.89% prevalence of PHPT among patients with Hashimoto's thyroiditis—significantly higher than the 0.3% prevalence in the general population 3.
This association is particularly common in elderly women and may reflect shared autoimmune mechanisms or common genetic predisposition rather than direct causation 3, 4.
Polycythemia (Hemoglobin 6.2, Hematocrit 50.4)
Hypothyroidism does NOT cause polycythemia; in fact, it typically causes anemia. The elevated hematocrit requires investigation for:
- Secondary polycythemia from chronic hypoxia, smoking, sleep apnea, or other causes
- Polycythemia vera (though less likely given the clinical context)
- Dehydration or hemoconcentration
Immediate Diagnostic Workup Required
For Hypercalcemia Evaluation
Measure intact parathyroid hormone (PTH) immediately—this is the single most important test to distinguish PTH-dependent from PTH-independent causes 2.
If PTH is elevated or inappropriately normal (>20 pg/mL) with hypercalcemia, this confirms primary hyperparathyroidism 5.
Measure 25-hydroxyvitamin D levels to exclude vitamin D deficiency as a secondary cause of elevated PTH, aiming for levels >20 ng/mL 5.
Check ionized calcium to confirm true hypercalcemia and avoid pseudo-hypercalcemia from albumin abnormalities 5, 2.
Obtain 24-hour urine calcium or spot urine calcium/creatinine ratio to assess urinary calcium excretion and help distinguish PHPT from familial hypocalciuric hypercalcemia 5.
Measure serum creatinine and eGFR to assess kidney function, as impaired renal function (GFR <60 mL/min/1.73 m²) is a surgical indication in confirmed PHPT 5.
For Polycythemia Evaluation
- Assess for secondary causes: smoking history, sleep apnea symptoms, chronic lung disease, cardiac right-to-left shunts
- Check erythropoietin level to distinguish primary from secondary polycythemia
- Evaluate hydration status and repeat complete blood count after adequate hydration
Management Algorithm
If Primary Hyperparathyroidism is Confirmed
Refer to both an endocrinologist and an experienced high-volume parathyroid surgeon for evaluation 5.
Surgical indications include 5:
- Corrected calcium >1 mg/dL above upper limit of normal (this patient is borderline at 10.4 mg/dL)
- Age <50 years
- Impaired kidney function (eGFR <60 mL/min/1.73 m²)
- Osteoporosis (T-score ≤-2.5 at any site)
- History of nephrolithiasis or nephrocalcinosis
For patients not meeting surgical criteria or declining surgery 5:
- Maintain normal calcium intake (1,000-1,200 mg/day)
- Ensure 25-hydroxyvitamin D >20 ng/mL with supplementation if needed
- Monitor serum calcium every 3 months
- Avoid high or low calcium diets
Thyroid Management
Continue or initiate thyroid hormone replacement to normalize TSH, as this is standard management for Hashimoto's thyroiditis with elevated TSH 6.
- Correcting hypothyroidism may modestly improve calcium levels through enhanced bone uptake, but this will NOT resolve hypercalcemia if PHPT is present 1.
Critical Pitfalls to Avoid
Do not attribute the hypercalcemia to hypothyroidism alone—this outdated concept is based on case reports that failed to adequately exclude hyperparathyroidism before modern PTH assays were available 1.
Do not order parathyroid imaging before confirming the biochemical diagnosis—imaging is for surgical planning, not diagnosis 5.
Do not supplement with vitamin D until hypercalcemia is resolved, and discontinue all vitamin D therapy if serum calcium exceeds 10.2 mg/dL 5.
Do not assume the polycythemia is related to thyroid disease—investigate it as a separate entity requiring its own diagnostic workup.
The Bottom Line
This patient has three distinct conditions: Hashimoto's thyroiditis (confirmed by elevated TSH and clinical history), likely primary hyperparathyroidism (explaining the hypercalcemia), and polycythemia of unclear etiology. While these conditions may co-exist more frequently than expected by chance alone—particularly Hashimoto's and PHPT 3—they are not causally related and each requires appropriate investigation and management.