Lithium is widely used in the acute and prophylactic treatment of affective disorders. Lithium affects thyroid hormone metabolism via different mechanisms. In patients this leads to a compensatory increase in pituitary thyroid stimulating hormone (TSH) which usually maintains the euthyroid status. This is probably the reason for the relatively high prevalence of goitre in lithium-treated patients; however, the enlargement of the gland is only moderate in most cases. Due to its immunostimulating effects lithium may support the appearance of thyroid auto-antibodies and the development of thyroiditis, which may be the reason for a higher prevalence of hypothyroidism in patients receiving lithium. However, also cases of hyperthyroidism in such patients have been reported repeatedly. Therapeutic recommendations for the treatment of disturbances of thyroid function during lithium treatment are given.