A 26 year old woman with lithium induced thyrotoxicosis is reported. The thyrotoxicosis was associated with a non-tender diffuse goitre and a low radioiodine uptake by the gland. The thyrotoxicosis was reversible and remitted on withdrawal of the drug. The histopathological alterations of the thyroid glad were characterised by extensive follicular cell disruption with no lymphocytic infiltration. It is postulated that lithium might directly damage thyroid follicular cells and that subsequent release of thyroglobulin into the circulation might be a cause of transient thyrotoxicosis.