Lithium treatment and thyroid function. A survey of 237 patients in long-term lithium treatment

In a cross-sectional study of 237 patients treated with lithium for more than six months, 10 patients had serum thyroid stimulating hormone (TSH) values above 35 mU/l and concomitantly low triiodothyronine (T3) and abnormally low serum thyroxine (T4). Eight patients had TSH values of 10.0-34.9 mU/l, while T3 and T4 were within or close to the normal ranges. Most of the patients had TSH values of 0.9-9.9 mU/l and normal or close to normal T3 and T4 values. Three male patients had TSH below 0.9 mU/l, but normal T3 and T4 values. A retrospective investigation three years later showed that all patients with TSH above 35 mU/l had clinically diagnosed overt hypothyroidism and treated accordingly with either replacement therapy with thyroxine or discontinuation of the lithium treatment. The thyroid parameters normalized spontaneously in 2 out of 3 patients who had their lithium treatment discontinued, while one still needed replacement therapy 1 1/2 years after withdrawal of lithium. Five patients had been put on replacement therapy before the cross-sectional study. The frequency of overt hypothyroidism thus indicated a prevalence about ten times as high as should be expected. An overrepresentation of women was found in the group with TSH values above 10 mU/l, whereas the group with TSH values of 0.9-10.0 mU/l showed no such sex predomination. The age distribution was equal in all groups. TSH appeared to be the most efficient parameter in revealing undetected hypothyroidism in the present investigation of patients in long-term lithium treatment.