All red blood cell (RBC) indices (hemoglobin (Hb), RBC count, hematocrit (HCT), MCV, MCH, and MCHC) were significantly lower in patients with hypothyroidism compared to the control group (p ≤ 0.05), except for RDW, which was significantly higher in patients with hypothyroidism (p = 0.000). This evidence concerns the gene GSTM1 and hypothyroidism.