OBJECTIVE: The aim of our study was to create a preoperative "index" that could predict the likelihood of additional hyperfunctioning parathyroid glands and let the surgeon determine whether to wait for the intraoperative parathyroid hormone (ioPTH) or to explore further. BACKGROUND: During parathyroidectomy for primary hyperparathyroidism (PHPT), discovering a minimally "enlarged" parathyroid gland creates a dilemma for the surgeon regarding the need for further exploration. Although ioPTH testing can potentially solve this problem after a 20- to 30-minute period, several surgeons recognize that further operative exploration may be more effective. METHODS: We analyzed a prospective database of 1235 consecutive patients who underwent parathyroidectomy for PHPT at our institution between March 2001 and August 2010. The Wisconsin Index (WIN) was defined as the multiplication of preoperative serum calcium by preoperative parathyroid hormone (PTH). Patients were divided into 3 WIN categories: low (<800), medium (801-1600), and high (>1600). The utility of the WIN was then validated on a subsequent cohort of 216 patients. RESULTS: The median age of the patients was 61 years (range, 10-91), and 77% of the patients were female. The mean preoperative calcium and PTH levels were 11.0 ± 0 mg/dL and 127 ± 3 pg/mL, respectively. The mean WIN for the entire cohort was 1461 ± 38 and highly correlated with gland weight (P < 0.000001). A single adenoma was identified in 1000 patients (81%), double adenoma in 100 patients (8%), and hyperplasia in 135 patients (11%). The mean gland weights for the 3 WIN catagories were low = 370 ± 33 mg, medium = 532 ± 39 mg, and high = 985 ± 28 mg, respectively (P < 0.000001). A WIN nomogram, consisting of the combination of WIN and parathyroid gland weight, accurately predicted the likelihood of additional hyperfunctioning parathyroid glands. For example, for a WIN of less than 800 and a gland weight of 500 mg, there is a 9% chance for additional hyperfunctioning parathyroid glands based on the WIN nomogram. In contrast, for the same gland weight, if the WIN is 801 to 1600, these chances increase to 28%, and if the WIN is more than 1600, the chance of multigland disease is 61%. Comparison between the predicted chances for another gland with the actual chance in the validation cohort identified an R value of 0.96. CONCLUSIONS: The WIN nomogram predicts the likelihood of additional hyperfunctioning parathyroid glands during parathyroidectomy. This simple intraoperative tool may be used to guide the decision of whether to wait for ioPTH results or to proceed with further neck exploration. Copyright © 2012 by Lippincott Williams & Wilkins.