REN and postural orthostatic tachycardia syndrome: Renin activity, expressed as angiotensin I generation, was decreased in patients with POTS (median, 3406 ng/mL; interquartile range (IQR), 2275–4767) when compared to healthy controls (median, 9949 ng/mL; IQR, 3937–17175; p < 0.001 (Figure 2).