Vitellogenin (VTG) is an egg yolk-precursor protein that serves as a nutrient source for developing embryos in oviparous vertebrates. The hormonal control of this protein has been studied in a variety of taxa, but details about the dynamics of this protein remain to be elucidated in sea turtle species. To investigate the dynamics of VTG in a multi-clutch species under natural conditions, 38 adult Loggerhead females entrained in the Florida Power and Light St. Lucie Nuclear Plant intake canal in Hutchinson Island, Florida were sampled from May–August of 2014. Blood samples were drawn to measure testosterone, estradiol 17β, and vitellogenin (T, E₂, and VTG, respectively) using enzyme-linked immunosorbent assays. Ultrasound imaging of the gonads was used to determine ovarian status and to measure ovarian follicle and oviductal egg size. Results showed that VTG concentration increased from May (8.27 mg mL⁻¹) to June (15.37 mg mL⁻¹) and declined into July and August (9.44 mg mL⁻¹); this decline corresponded with the end of the nesting season. E₂ declined from 718.02 pg mL⁻¹ in May to 95.89 pg mL⁻¹ in July–August, and T declined from 2,008.35 pg mL⁻¹ in May to 1,221.24 pg mL⁻¹ in July–August. Mean concentration for both gonadal steroids was significantly higher in reproductively active females than means of reproductively inactive females, though overlapping concentrations of the steroids occurred between active and inactive animals. However, VTG concentration was high in reproductively active turtles and undetectable in gonadally quiescent turtles. We concluded that the addition of VTG measurement in conjunction with the gonadal steroids provides a more accurate and easily interpretable way to predict reproductive status of adult Loggerhead females. Finally, gonadal steroid and VTG concentration in our study corresponded only with late nesting animals, indicating that early season females do not become entrained in the intake canal of the power plant.