OBJECTIVES: Hypoparathyroidism (HypoPARA) is an endocrine disorder in which the parathyroid glands produce insufficient parathyroid hormone. This study aimed to 1) estimate the number of insured HypoPARA patients in the United States (US), and 2) obtain physician assessment of disease severity. METHODS: Prevalence was estimated through diagnoses of HypoPARA in the IMS LifeLink Health Plan Claims Database, containing 60 million unique patients, over a 12-month period from October 2007 through September 2008, and projected to the US insured population. Incidence was also calculated by counting the total number of parathyroidectomy, thyroidectomy, and neck dissection surgeries in the same database.  Surgeries resulting in either transient or chronic (>6 months) HypoPARA were counted to determine proportion of surgeries resulting in HypoPARA. A physician primary market research study was conducted to assess disease severity and determine the percentage of new non-surgical patients. Incidence was entered into a model to derive estimated prevalence. RESULTS: The prevalence approach estimated 65,325 insured US HypoPARA patients, of which 90% are chronic. The incidence approach yields 117,342 relevant surgeries resulting in 8,901 cases of HypoPARA over a 12 month period. Overall, 7.6% of surgeries resulted in HypoPARA, of which 75% are transient and 25% are chronic. When entered into the model, the estimated prevalence of insured patients is 65,389. The physician market research found that among all thyroidectomies and parathyroidectomies and neck dissections performed in a year, 26% resulted in transient HypoPARA and 5% progress to a chronic state. Of the HypoPARA patients treated over the previous 12 months, 75% were due to surgery. Physicians categorized their patients as 43% mild, 39% moderate, and 18% severe HypoPARA. CONCLUSIONS: Two methods yielded similar estimates of the number of insured HypoPARA patients in the US (65000). Extrapolated to the US population, the number of HypoPARA patients is estimated at 78,000.