Increased risk of hyperthyroidism among patients hospitalized with bipolar disorder

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Abstract
Hyperthyroidism has been associated with affective disorder in many cross-sectional studies, but longitudinal studies in this connection are scarce. We assessed whether hospitalization with depressive disorder or bipolar disorder was a risk factor for development of hyperthyroidism. We conducted a historical cohort study using the Danish register data. The observational period was 1977--99. Three study cohorts were identified: all patients with a first hospital admission with resulting index discharge diagnoses of depression, bipolar disorder, or osteoarthritis. The risks of subsequently being readmitted with a resulting discharge diagnosis of hyperthyroidism were estimated in survival analyses. A study sample of 133,570 patients discharged with an index diagnosis was identified. Exactly 610 patients were later readmitted following diagnoses of hyperthyroidism. Patients with depressive disorder did not have an increased risk of hyperthyroidism, whereas patients with bipolar disorder had an increased of risk on the margin of statistical significance, when compared to patients with osteoarthritis. Patients with bipolar disorder had a significantly increased risk of hyperthyroidism when compared to patients with depression. The results apply only to hospitalized patients. Diagnoses are not validated for research purposes. Patients hospitalized with bipolar disorder tend to be at greater risk of readmission with hyperthyroidism than suitable control patients.