Platelets in Hyperthyiroidism: Studies on Platelet Counts, Mean Platelet Volume, 111-Indium-Labeled Platelet Kinetics, and Platelet-Associated Immunoglobulins G and M*

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Abstract
We compared in 15 patients with hyperthyroidism (11 with Graves' disease, 3 with toxic adenoma, and 1 with multinodular goiter) platelet counts (PC) and mean platelet volume (MPV) before and 3 weeks after initiation of antithyroid drug therapy when the patients were euthyroid. In addition, platelet kinetic studies of autologous 111-indium-labeled platelets and platelet-associated immunoglobulins G and M (PAIgG and PAIgM, respectively) were investigated. The control group for the platelet kinetic studies consisted of 2 patients with diffuse nontoxic goiter and 86 patients who were studied for evaluation of their arteriosclerotic vascular disease. After 3 weeks of antithyroid drug therapy there was a significant increase in PC and a decrease in MPV compared with the pretreatment values (pretreatment PC median, 215 × 109/L; range, 96–350 × 109/L; PC median 3 weeks later, 248 × 109/L; range, 157 – 384 × 109/L; P < 0.005; pretreatment MPV median, 10.6 fL; range, 9.1–13.2 fL; MPV 3 weeks later, 9.9 fL; range, 8.4–11.0 fL; P < 0.005). Ill-Indium platelet recovery was normal in all subjects. Plateletlifespan was significantly shortened in the patients with hyperthyroidism compared with the control group (median, 163.8 h; range, 128.5–206 h; vs. 180.0 h; range, 138.3–231.5 h; P< 0.05). Platelet turnover averaged 45.6 (range, 25.6–71.9) × 109/L·day; values above the limit of normal were found in 7 of 15 patients with hyperthyroidism. Three patients with Graves' disease had elevated levels of PAIgG; 1 of these patients had elevated levels of PAIgM and was the only patient with thrombocytopenia (PC, 96 × 109/L). Various combinations of statistical correlations between the degree of hyperthyroidism, pretreatment PC and MPV, platelet kinetic studies, levels of PAIg, and serum levels of antithyroid antibodies revealed no significant differences. These findings suggestthat the platelet changes observed in hyperthyroidism, such as lower PC and increased MPV, together with the shortened platelet lifespan reflect metabolically rather than immunologically mediated phenomena, although these may be involved in cases with marked thrombocytopenia.