Indocyanine green alters intraoperative parathyroid hormone measurements: a timing-dependent effect

Background Intraoperative indocyanine green (ICG) angiography and intraoperative parathyroid hormone (ioPTH) monitoring are widely used in thyroid surgery, yet analytical interference by ICG remains poorly characterized. This study quantitatively evaluated ICG-induced immunoassay distortions. Methods The study included paired in vitro spiking experiments (n = 6) and an in vivo thyroidectomy cohort (n = 6) with serial sampling during paused surgical dissection. Exploratory cross-platform analysis included paired perioperative samples from one patient and additional in vitro testing of three independent specimens. Results ICG caused significant false-positive elevations. In vitro , the median of individual fold-increases was 5.98 for PTH (1–84) and 3.34 for i-PTH (p = 0.031), producing an abnormal diagnostic ratio inversion (PTH (1–84)/i-PTH > 1.0). In vivo , measured levels peaked at 3 minutes post-injection (234.8% of baseline for 1–84 PTH; 146.1% for i-PTH) and approached baseline within 20 to 30 minutes (p > 0.05). Conclusion Intravenous ICG can transiently distort ioPTH measurements and may lead to misleading interpretation during thyroid surgery. Whenever feasible, baseline ioPTH should be obtained before ICG administration. The observed return toward baseline within approximately 20 to 30 minutes is preliminary and requires validation before a post-ICG sampling interval can be recommended.