Patients with biochemically confirmed primary hyperparathyroidism and non-localizing SPECT-CT exam within the past year will be included. Subjects will be treated with calcitonin to lower calcium levels immediately prior to reimaging. The goal of this study is to determine whether lowering calcium will improve uptake/retention of sestamibi and improve sensitivity of SPECT-CT to localize parathyroid adenoma.

Inclusion Criteria:

• Diagnosis of primary hyperparathyroidism
• Non-localizing SPECT-CT performed within 365 days prior to consent to participate in study
• Patient desires surgical intervention for treatment of PHPT
• No contraindications to 99mTC-Sestamibi
• No contraindications to treatment with calcitonin
• Serum calcium level prior to non-localizing SPECT-CT is ≥10.5 mg/dL
• Patient consents to participate and undergo second SPECT-CT for purposes of research

Exclusion Criteria:

• Previous surgery to the neck, including resection of parathyroid tissue, except where end organ damage is present and further surgical intervention is medically necessary
• Contraindication to 99mTC-Sestamibi SPECT-CT as evidenced by allergic reaction or adverse event during index SPECT-CT
• Allergy to calcitonin
• Hypocalcemia (contraindication to calcitonin)
• Vitamin D deficiency (contraindication to calcitonin)
• Previous treatment with radioactive iodine
• New prescription of thyroid medication (levothyroxine, armour thyroid tablets, etc. must be taken at time of index scan and research scan)
• Lithium exposure within one year of SPECT-CT (index and research scans)
• Secondary hyperparathyroidism
• Benign familial hypocalciuric hypercalcemia
• Known malignancy, particularly multiple endocrine neoplasia
• New prescription of thiazide diuretic, (thiazide diuretic must have been taken at the time of index scan and second scan)
• Currently taking calcium channel blockers
• Pregnancy