ObjectiveTo determine whether headache disorders are a risk factor for the development of new onset hypothyroidism. BackgroundPast studies have reported associations between headache disorders and hypothyroidism, but the directionality of the association is unknown. MethodsThis was a longitudinal retrospective cohort study using data from the Fernald Medical Monitoring Program (FMMP). Residents received physical examinations and thyroid function testing every 3 years during the 20 year program. Residents were excluded from the cohort if there was evidence of past thyroid disease or abnormal thyroid function tests at the first office visit. A diagnosis of a headache disorder was established by self-report of frequent headaches, use of any headache-specific medication, or a physician diagnosis of a headache disorder. The primary outcome measure was new onset hypothyroidism defined as the initiation of thyroid replacement therapy or TSH 10 without thyroid medication. A Cox survival analysis with time dependent variables were used for the model. Headache disorders, age, sex, body mass index, income, smoking, narcotic use, and hypothyroidism-producing medications were independent variables in the model. ResultsData from 8412 residents enrolled in the FMMP were used in the current study. Headache disorders were present in about 26% of the residents and new onset hypothyroidism developed in approximate to 7%. The hazard ratio for the development of new onset hypothyroidism was 1.21 (95% CI=1.001, 1.462) for those with headache disorders. ConclusionsHeadache disorders may be associated with an increased risk for the development of new onset hypothyroidism.