Background: Apelin and its receptor are located in hypothalamo-pituitary axis and inhibit TSH secretion. Insulin resistance (IR) is linked to apelin, thyroid function, and polycystic ovary syndrome (PCOS). Apelin levels in obese and non-obese PCOS women display contradictory results. Objective: To compare serum apelin in IR PCOS and non-IR PCOS groups, evaluate the cross-talk of apelin, IR, thyroid function, and cardiovascular risk factors in both groups, and identify the diagnostic value of apelin for IR in PCOS. Patients and Methods: The study included 60 young non-obese euthyroid PCOS women: IR PCOS (n=28) and non-IR PCOS (n=32), and age-matched healthy fertile groups. Serum apelin-36, calculated  homeostatic model assessment–IR (HOMA-IR), and thyroid hormones were evaluated. Results: IRPCOS had significantly higher apelin compared to non-IR PCOS and healthy control groups with insignificant differences between the two latter groups. The bivariate analysis demonstrated a significant positive correlation between apelin and glucose, insulin, HOMA-IR, atherogenic lipid profile, and TSH and a significant negative correlation with free T4 in IR PCOS while inverse relations were observed in non-IR PCOS. After controlling for HOMA-IR, only HDL in IR PCOS and TSH and cholesterol in non–IR PCOS preserved the significant correlations. Cut-off value≥ 39 pg/dl of apelin showed high specificity (88%) and sensitivity (86%) to identify IR in PCOS Conclusion: Serum apelin is higher in IR than in non-IR PCOS patients. The relations between apelin and thyroid function, and cardio-metabolic risk factors are IR dependent. Apelin is a marker of IR in PCOS and had a direct talk with TSH, HDL-c, and cholesterol.