A retrospective, observational cohort study of 102 consecutive patients (125 shoulders) with calcific tendinitis is presented. Of the patients, 73 (71.6%) were women and 29 (28.4%) were men. Compared with population prevalences, significant levels of endocrine disorders were found. We compared 66 patients (62 women [93.9%] and 4 men [6.1%]; mean age, 50.3 years) (81 shoulders) with associated endocrine disease with 36 patients (11 women [30.6%] and 25 men [69.4%]); mean age, 52.4 years) (44 shoulders) without endocrine disease. The endocrine cohort was significantly younger than the non-endocrine cohort when symptoms started (mean, 40.9 years and 46.9 years, respectively), had significantly longer natural histories (mean, 79.7 months compared with 47.1 months), and had a significantly higher proportion who underwent operative treatment (46.9% compared with 22.7%). Disorders of thyroid and estrogen metabolism may contribute to calcific tendinitis etiology. Classifying calcific tendinitis into type I (idiopathic) and type II (secondary or endocrine-related) aids prognosis and management.