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Background: Roughly three percent of episodic migraine patients evolve into the most burdensome chronic form of this condition every year. While some of the determinants behind this transformation are well established, others are still ill defined. Hypothyroidism is a prevalent endocrinological disorder that can both produce a secondary headache or aggravate a pre-existing primary headache disorder such as migraine. Objective: We aimed to re-assess the association between hypothyroidism and chronic migraine controlling for factors such as hormone replacement treatment status and bodyweight. Methods: We retrospectively analyzed the medical records of episodic and chronic migraine patients who consecutively consulted our headache clinic in order to determine the prevalence of adequately treated hypothyroidism in each group. Only patients receiving a stable dose regimen were included. The body mass index and other possibly confounding covariates were also collected. Results: Data from 111 migraine patients was included for analysis. Most (88.6%) of chronic migraine sufferers were overusing acute medication. Treated hypothyroidism was significantly more prevalent in chronic migraine patients (29.55%) compared to episodic migraine patients (8.96%). This association was independent of the patients' body mass index or other variables. Conclusion: Alterations of neuronal metabolism, deficient calcitonin release, or focal inflammation causing local hormonal deactivation might explain why hypothyroidism, in spite of levothyroxine replacement therapy, is associated with migraine chronification. Further studies evaluating these factors are warranted. [ABSTRACT FROM AUTHOR]