Risk of agranulocytosis and aplastic anaemia in relation to use of antithyroid drugs. International Agranulocytosis and Aplastic Anaemia Study.

The relation of the use of antithyroid drugs to the risk of developing agranulocytosis and aplastic anaemia was evaluated in a population based case-control study with patients from Israel and seven regions in Europe. Data were obtained from cases and hospital controls by interview. Use of antithyroid drugs in the week before the onset of illness was compared in 262 patients with agranulocytosis and 1771 controls. Forty five patients (17%) and five controls (0.3%) had used antithyroid drugs. The relative risk was estimated to be 102 (95% confidence interval 38 to 275) taking into account confounding by other factors, including the use of other drugs. The excess risk for use of antithyroid drugs in any one week was estimated to be 6.3 cases of agranulocytosis per million users. Use of antithyroid drugs in a five month period ending one month before admission to hospital was compared in 135 patients with aplastic anaemia and 2145 controls. Four patients (3%) and five controls (0.2%) had taken drugs; the estimate of relative risk was 9.2 (95% confidence interval 1.8 to 47) after control for confounding. The estimate of excess risk of agranulocytosis with the use of antithyroid drugs was lower than found previously. Although the excess risk for aplastic anaemia was not calculated, these data suggest that it is very low.