Pancytopenia in the first trimester is very rare. A 33-year-old multiparous woman presented with nausea, loss of appetite, and bodyweight loss of 7.4 kg at 91/7 weeks of gestation due to hyperemesis gravidarum. Her laboratory data demonstrated pancytopenia involving white blood cell count of 3500/μL, a hemoglobin level of 9.8 g/dL, and a platelet count of 10.5 × 104/μL. An extensive investigation into the causes of the pancytopenia detected true hyperthyroidism: thyroid-stimulating hormone, <0.02 μU/mL; free triiodothyronine, 11.25 pg/mL; free thyroxine, 4.74 ng/dL; and anti-thyroid-stimulating hormone receptor antibodies, 12.2 IU/L. Propylthiouracil was started at a dose of 300 mg/day at 105/7 weeks of gestation, which resulted in the normalization of her blood parameters and concomitant improvements in her free triiodothyronine and free thyroxine levels at 120/7 weeks of gestation. Pancytopenia in the first trimester might be indicative of hidden hyperthyroidism.