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 thyro idectomy: ten patients (group A) underwent whole-body scintigraphy with 185 MBq of I-131 and the other ten (group B) received 3700 MBq of I-131 therap y, All patients were in a hypothyroid state at the time of administration o f I-131 and started L-thyroxine 150 mu g/day 3 days after I-131 administrat ion. Free and bound triiodothyronine and thyroxine, thyroid-stimulating hor mone, thyroglobulin, thyroglobulin antibodies, thyroid peroxidase/microsoma l antibodies, white blood cell, lymphocyte counts and lymphocyte subsets we re serially determined at baseline and at days 2, 7, 15, 30 and 60 after 13 1I administration. Twenty healthy age- and sex matched individuals were use d 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. I n group B, patients showed a delayed reduction in the total lymphocyte coun t at days 15, 30 and 60 (P=0.008, 0.004 and 0.018, respectively), and a dec rease in B cells throughout the study (at days 7, 15, 30 and 60: P=0.006, 0 .0017, 0.0017 and 0.0017 respective ly). A transient decrease in NK cells w as observed at days 15 (P=0.025) and 30 (P=0.008). Among T cells, the helpe r phenotype (CD4+) was mainly affected,, resulting in a reduction in the CD 4+/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 cli nical sign of immunosuppression during the follow-up period. In patients wi th thyroid cancer the sensitivity of lymphocytes to the effects of I-131 ad ministered for diagnostic or therapeutic purposes depends upon lymphocyte p henotype and I-131 activity, NK cells are the most radiosensitive cells, be ing reduced even by low I-131 activity. At higher activity all subtypes sho w 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 immunosuppression.