Radioiodine-induced changes in lymphocyte subsets in patients with differentiated thyroid carcinoma

This study evaluated changes in lymphocyte subsets in patients with thyroid carcinoma who received iodine-131 for diagnostic and therapeutic purposes. Twenty thyroid cancer patients were entered in the study after total thyroidectomy: ten patients (group A) underwent whole-body scintigraphy with 185 MBq of ¹³¹I and the other ten (group B) received 3700 MBq of ¹³¹I therapy. All patients were in a hypothyroid state at the time of administration of ¹³¹I and started L-thyroxine 150 μg/day 3 days after ¹³¹I administration. Free and bound triiodothyronine and thyroxine, thyroid-stimulating hormone, thyroglobulin, thyroglobulin antibodies, thyroid peroxidase/microsomal antibodies, white blood cell, lymphocyte counts and lymphocyte subsets were serially determined at baseline and at days 2, 7, 15, 30 and 60 after ¹³¹I administration. Twenty healthy age-and sex-matched individuals were used as a reference population for lymphocyte subset values. In group A only a reduction in NK cells at days 7 (P = 0.043) and 15 (P = 0.037) was observed. In group B, patients showed a delayed reduction in the total lymphocyte count at days 15, 30 and 60 (P = 0.008, 0.004 and 0.018, respectively), and a decrease in B cells throughout the study (at days 7, 15, 30 and 60: P = 0.006, 0.0017, 0.0017 and 0.0017 respectively). A transient decrease in NK cells was observed at days 15 (P = 0.025) and 30 (P = 0.008). Among T cells, the helper phenotype (CD4+) was mainly affected, resulting in a reduction in the CD4+/CD8+ ratio at day 60 (P = 0.046). Comparing the two groups, the numbers of B lymphocytes at day 30 (P = 0.023) and NK cells at days 2 (P = 0.037) and 30 (P = 0.023) were significantly lower in group B. Neither group showed any clinical sign of immunosuppression during the follow-up period. In patients with thyroid cancer the sensitivity of lymphocytes to the effects of ¹³¹I administered for diagnostic or therapeutic purposes depends upon lymphocyte phenotype and ¹³¹I activity. NK cells are the most radiosensitive cells, being reduced even by low ¹³¹I activity. At higher activity all subtypes show a reduction, which is more marked and prolonged for B lymphocytes and, to a lesser extent for T-helper lymphocytes. These changes do not result in clinically relevant immunosuppnssion.