Background: Hypothyroidism and frozen shoulder (FS) have been associated, although this relationship remains uncertain. The main objective of this study was to determine the prevalence of hypothyroidism in patients with FS. Methods: A case-control study was performed to compare FS patients (cases) with patients who visited an orthopedic service for other clinical conditions (controls). FS was diagnosed according to specific criteria based on anamnesis, physical examination, and shoulder radiographs. A specific questionnaire was applied, and measurements of serum thyroid-stimulating hormone (TSH) and free tetraiodothyronine were performed in all subjects. Results: We evaluated 401 shoulders from 93 FS patients and 151 controls. The prevalence of hypothyroidism diagnosis was significantly higher in the FS group (27.2% vs. 10.7%; P =.001). There was also a tendency for higher prevalence of bilateral FS among patients with elevated TSH levels (P =.09). Mean serum TSH levels were higher in patients with bilateral FS compared with those with unilateral compromise (3.39 vs. 2.28; P =.05) and were higher in patients with severe FS compared with those with mild and moderate FS together (3.15 vs. 2.21; P =.03). Multivariate analysis showed that FS was independently related to a diagnosis of hypothyroidism (odds ratio, 3.1 [1.5-6.4]; P =.002). There was a trend toward independent association between high serum TSH levels and both severe (odds ratio, 3.5 [0.8-14.9]; P =.09) and bilateral (odds ratio, 11.7 [0.9-144.8]; P =.05) compromise. Conclusion: The prevalence of hypothyroidism was significantly higher in FS patients than in controls. The results suggest that higher serum TSH levels are associated with bilateral and severe cases of FS. (C) 2017 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserved.