Effect of 131I on the anemia of hyperthyroidism

Data from the National Thyrotoxicosis Therapy Follow-Up Study (NTTFS) are presented here to document the existence of anemia in hyperthyroidism, a mild and reversible anemia that is simultaneously ameliorated with reversal of the hyperthyroid state. Among 20,600 women entered into the NTTF study with no previous history of hematological disorders, the prevalence of anemia was found to range from 10-15%, appearing to be higher in those selected for treatment with 131I when compared to those selected for surgery. An attempt is made to verify the recent hypothesis that thyroid hormone levels in the supraphysiologic range may suppress erythrogenesis. Two statistically significant regression models are consistent with a hypothesis of thyrotoxic bone marrow suppression. However, both associations are weak enough to suggest that some other physiologic improvement underlies the amelioration of anemia when hyperthyroidism is reversed. The degree of improvement in hematological status is similar for women in both treatment groups. Among 4464 women for whom serial hematological tests are obtained, over 3/4 of anemic patients are no longer anemic after an average 6.2 yr of follow-up. Clinicians are reassured that radioactive iodine exposure causes no further insult to the bone marrow, no matter what the cumulative dosage. The highly fractionated low dose bone marrow exposures to radiation account for the minimal hematological risks of 131I treatment.