Utilization of frequency locking for tracking optical resonances of microresonators is an emergent technique, which has relevance in label-free biosensing, owing to its extremely high sensitivity in detecting adsorption on a microcavity’s surface. In this study, we demonstrate the capabilities of a technique known as FLOWER (Frequency Locked Optical Whispering Evanescent Resonator) in a real-life problem: the detection of human chorionic gonadotropin (hCG) in urine samples. hCG, besides being a hormone secreted during pregnancy, is a performance enhancing agent prone to be abused by athletes, and is routinely investigated in doping laboratories. The gold standard in hCG detection is mass spectrometry. As the lowest limit of detection by mass spectrometry is close to the cut-off value for a positive result, various other measurement techniques such as optical interferometry, photoluminescence, or electrochemical sensors have been used to try to enhance measurement sensitivity. These methods, however, do not show a significant improvement in the limit of detection over mass spectrometry and they mostly have a narrow detection range. In addition, measurement selectivity is another issue. Here, we use microtoroidal optical resonators functionalized with antibodies against hCG, and track their resonances using frequency locking upon analyte infusion. Urine samples from pregnant women were measured using our setup. Our results demonstrate that FLOWER can detect the presence of hCG in a large concentration range from 1 fM to 10 nM.