Effect of therapy on the neutropenia of hyperthyroidism.

Observations in a patient with recurrent hyperthyroidism, each time associated with neutropenia which resolved after therapy, prompted a chart review of other patients referred for radioactive iodine therapy. Of 99 untreated patients, 18 had neutrophil counts of less than 2,000/cu mm. After therapy with either thionamides or 131I, 41 of 53 (77%) evaluable patients had an increase in neutrophil count. Eleven of these evaluable patients had neutropenia before therapy; after therapy, all 11 had an increase in their neutrophil counts into the normal range, with a mean increase of 170%. In one patient, studies on the mechanism of neutropenia indicate that bone marrow production and reserve remain normal and that circulating neutrophils are normally marginated. A decreased neutrophil circulation time may be the cause of neutropenia associated with hyperthyroidism.