Hypothyroidism is common with lithium therapy. Thyrotoxicosis is rare. Of 81 cases of thyrotoxicosis related to lithium therapy reported 22 were silent thyroiditis , 39 Graves' disease and 11 autonomous nodule.
We report a 56 year old male who had been on lithium for the last four years for bipolar illness, was referred to us when he was found to be biochemically thyrotoxic on routine testing. The TSH was less than 0.01, FT37.3, FT4 154. On his visit to the clinic he denied any symptoms suggestive of thyrotoxicosis, neither did his GP mention any suggestive symptoms. No history of sore throat or ingestion of idine containing compounds were present. On examination he had a pulse rate of 80/min, regular, no eye signs and there was no palpable thyroid mass. TFT revealed TT4 97, FT35.8, FT4 17 and a TSH of 0.04. His TPO antibody was 13 and TSH receptor antibody was negative. Thyroid scintiscan revealed an uptake of 0.75% which is at the lower end of normal,( normal 0.5% to 4.0%).2 months later on follow up the patient remains clinically euthyroid . TFT- TSH 21.3, TT4 69.
We propose the diagnosis of our patient to be silent thyroiditis. As described by Woolf, silent thyroiditis is characterised by spontaneously resolving thyrotoxicosis, a very low radioiodine uptake and often followed by a hypothyroid phase.
The prevalence of Lithium associated thyrotoxicosis has been estimated at 1.7-2.5%, and an incidence of thyrotoxicosis to be 2.7per 1000 patient year treated with lithium. Lithium induced or lithium exacerbated autoimmune thyroiditis is the likely explanation for silent thyroiditis in the lithium treated population. The other possibility is direct toxic effect of lithium on thyroid. Intrathyroidal lithium concentrations are much higher than those in blood.Biopsy reports to date have shown to have lymphocytic infiltration in one patient only and the four other biopsy reports did not reveal any immunologic evidence. Thyrotoxicosis may be missed because of transient and asymptomatic nature of the illness.