Green sea turtles (Chelonia mydas) have historically had markedly inverted calcium: phosphorous (Ca:P) ratios on prerelease plasma biochemistries at a rehabilitation center in North Carolina. A clinical diagnostic investigation was conducted to assess ionized calcium (iCa), 25-hydroxyvitamin D (Vit D), and parathyroid hormone (PTH) levels in a subset of the affected animals (n = 10). Reference values for iCa, Vit D, and PTH have not been reported for wild green sea turtles in this region; therefore, healthy wild turtles (n = 10) that were incidentally captured in commercial fishing nets along the North Carolina coast were also evaluated. Median (10th–90th percentiles) levels of iCa, PTH, and Vit D for turtles in rehabilitation were 0.63 (0.55–0.72) mmol/L, 2.95 (1.31–3.96) pmol/L, and 27.5 (17.2–64.6) nmol/L, respectively, and for wild turtles were 1.05 (0.87–1.23) mmol/L, 0.75 (0.04–2.28) pmol/L, and 36 (16.1–72.1) nmol/L, respectively. Significant differences were found between the two populations for Ca, P, Ca:P, iCa, and PTH. Vitamin D values did not differ between the two populations, suggesting that turtles in rehabilitation maintain adequate levels. Calcium and iCa values were significantly lower and PTH and P were significantly higher in turtles in rehabilitation than in wild turtles. This suggests that dietary requirements are not being met in the captive population. The values presented here for iCa, Vit D, and PTH from the healthy wild population may serve as references for green sea turtles in rehabilitation.