Incidence and causes of hyperthyroidism in blacks.

Between 1974 and 1984 688 black patients with hyperthyroidism were seen at one clinic. Graves' disease made up 88% of cases, with a female: male ratio of 7,9:1. The average annual incidence for females was 8,75/100,000 and for males 0,70/100,000. There appeared to be a progressive increase in incidence (60%) over the period. A bimodal age distribution was observed with peaks at 35-54 years (almost exclusively Graves' disease) and in those older than 64 years (Graves' disease 66%, nodular goitres 34%). Although hyperthyroidism seems rare in rural Africa, 59% of the patients surveyed (N = 88) had been born in such an area and were urban migrants. Moreover, some ethnic groups were disproportionately represented. In one of these (Tswana-speaking), 81% had migrated from rural areas of endemic goitre, usually as adults. It is concluded that hyperthyroidism remains relatively uncommon in the black population in this region. The incidence of Graves' disease, but not of toxic nodular goitre, appears to be increasing rapidly. The process of urbanisation, including possible increases in dietary iodide, may contribute to this apparent increase in Graves' disease in urban areas.