Many individuals experience seasonal changes in mood and behavior. Various theories have been suggested to explain the mechanisms of these changes. However, the mechanisms of seasonal mood and behavioral changes remain unclear. The author suggests that brain thyroid hormones may play an important role in seasonal changes in mood and behavior. This suggestion is based on the facts that seasonal changes in light and temperature may affect the metabolism of brain thyroid hormones and that small alterations of the brain thyroid economy, independent of peripheral changes in thyroid status, may produce significant behavioral effects. The author further suggests that there may be a fault in the thyroid metabolism in the brain in seasonal affective disorder patients, and that fault cannot be identified by studying the peripheral thyroid hormone metabolism. Seasonal mood and behavioral changes may also be related to the interaction between thyroid hormones and different neurotransmitter systems in the brain.