Thyroid function was assessed in a prospective survey of 13 manicdepressive patients before and after 3 months on lithium carbonate and in a further 12 patients who had received lithium for 20 months. There was a significant increase in thyroid size as measured by quantitative scintiscanning in 8 patients in the first group. One male in the second group had a goitre. There was a rise in plasma TSH in the first group and a significant fall in saliva to plasma iodide. It is suggested that pathogenesis of lithium induced goitre is related to a disturbance in the iodide concentrating mechanism. Thyroid status should be evaluated in patients who are suitable for lithium therapy.