Lithium is commonly used to treat bipolar affective disorder. It is well known to adversely affect thyroid function, most commonly causing hypothyroidism. Hyperthyroidism is a rare complication. Similarly, total body irradiation (TBI) associated with BMT is well known to affect thyroid function frequently causing hypothyroidism. Hyperthyroidism secondary to TBI is unusual. To have both a history of chronic lithium therapy and TBI with a BMT in patient presenting with hyperthyroidism is an extremely atypical situation. We describe a 39-year-old male who presented with primary hyperthyroidism after 18 years of lithium use for bipolar affective disorder and 6 years post-BMT for AML-M4.