A substance present in the blood that, on being filtered through glomeruli of the kidney, is reabsorbed by the tubules up to a certain limit, that being the upper limit of the concentration of the substance in normal plasma. High-threshold substances (e.g., chlorides or glucose) are entirely or almost entirely reabsorbed. Low-threshold substances (e.g., phosphates or urea) are reabsorbed in limited quantities. No-threshold substances (e.g., creatinine sulfate) are excreted entirely.