Objective: To evaluate the hormonal and hematologic parameters of 24 patients with hyperemesis gravidarum without evidence of thyroid disease compared with matched controls. Methods: Twenty-four pregnant women with hyperemesis and 20 control subjects were included in this study. A prospective comparison of hormonal milieu of hCG and thyroid function was performed. Results: Mean serum hCG, ft3, and fT4 levels of patients were significantly higher than those of controls (P <0.007), while there was no statistically significant difference in terms of TSH. Serum hCG correlated negatively with TSH and positively with ft3 and fT4 in the patient group (r=0.66, r=0.70, r=0.85; P <0.05, respectively), while there was no relationship between hCG and thyroid functions in controls (r<0.25). The lymphocyte count of patients was significantly higher than that of controls (P <0.007), while there were no statistically significant differences in the overall white blood count and the granulocyte count (P >0.007). The lymphocyte count correlated positively with serum hCG, fT4 and fT3, and negatively with TSH in the patient group (r=0.73, r=0.72, r=0.64 and r=-0.63; P <0.05, respectively), while there was no relationship between lymphocyte count and serum hCG, fT4, fT3 and TSH in controls (r <0.25). Conclusions: Maternal immune recognition of the conceptus and immune response, hypothetically, may be related to the high level of hCG and/or fT4. Patients who Rave hyperthyroidism in early pregnancy should be assessed for transient hyperthyroidism as it relates to hyperemesis gravidarum.