This study investigates the effectiveness of using postoperative phosphate levels and calcium/phosphate ratio (Ca/P) as indicators for parathyroid hormone (PTH) levels after total thyroidectomy. Conducted as a retrospective cohort study at a tertiary care hospital, it included 185 patients who underwent surgery between 2015 and 2021, excluding those with pre-existing conditions that could affect the results. The research aimed to find cost-effective alternatives for monitoring calcium post-surgery by comparing phosphate and Ca/P with PTH levels using Receiver operating characteristic (ROC) curves. The results showed that neither phosphate levels nor Ca/P ratios are reliable surrogates for PTH, with area under the curve (AUC) values ranging from 0.55 to 0.66 for phosphate and 0.61 to 0.79 for Ca/P, all below the threshold of 0.8 for a good screening test. This study highlights the need for further research to identify effective methods for postoperative calcium monitoring in thyroidectomy patients.