Aims: We aimed to assess the association between early onset of affective disorders and hypothyroidism in a larger number of psychiatric drug-free patients with bipolar disorder (BD) or major depressive disorder (MDD). Methods: Early onset of affective disorders was defined as BD and MDD developed before the age of 22 years. The hypothyroidism hierarchy were diagnosed biochemically and included subclinical and overt hypothyroidism in this study. Demographic and clinical data including diagnosis, illness duration and thyroid function parameters (TSH, T4, FT4, T3 and FT3) at admission were collected from patients' records. Adjusted odds ratio (OR) and 95% confidence interval (CI), which were estimated by logistic regression model, were used to assess the association between early-onset affective disorder and hypothyroidism. Results: The prevalence of hypothyroidism in early-onset affective disorders was higher than that in late-onset patients (in BD, 9.4% vs. 6.2%, chi 2 = 6.020, P = 0.014; and in MDD 12.7% vs. 6.6%, chi 2 = 13.295, P < 0.001). Early-onset affective disorders were 2.097 times (95% CI: 1.409-3.123) more likely to have hypothyroidism compared with late-onset patients, after adjustment for age, gender, duration of illness and mood episode (adjusted OR: 1.965, 95% CI: 1.198-3.221 in BD, and adjusted OR: 2.831, 95% CI: 1.378-5.817 in MDD, respectively). Limitation: Because of the cross-sectional design of this study, we were unable to sort out causality between earlyonset affective disorders and hypothyroidism. Conclusion: Early-onset affective disorder may be associated with higher prevalence of hypothyroidism.