Background: Seasonal Affective Disorder (SAD) is characterised by lowered mood and atypical depressive symptoms such as hypersomnia, weight gain and fatigue. These symptoms seem associated with hypothyroidism, but the results of evaluations of the thyroid function in SAD patients have been conflicting, most likely due to the very small number of observations. Methods: In total, 83 patients fulfilling the DSM-III-R criteria for SAD were treated with bright light for 1 week in an open trial. Thyroid function was evaluated by TSH (thyroid-stimulating hormone), T₄ (thyroxine) and T₃ (triiodthyronine) levels at baseline and after 1 week of bright light treatment. Results: The response rate in terms of a 50% reduction of pre-treatment scores on the Hamilton Depressions Rating Scale (HAM-D₁₇) was 61%. The TSH levels in all 83 patients decreased significantly from 1.57 at baseline to 1.30 at endpoint. In the group of responders (n=52) the TSH levels decreased significantly from 1.71 to 1.37, while in the group of non-responders (n=31) the decrease in TSH levels was not statistically significant. Conclusion: During 1 week of bright light therapy the TSH levels in SAD patients were reduced, with the highest reduction in the group of patients responding to light therapy.