Measurements of the isotopic composition of water used in the production of triple point of water cells show that state-of-the-art triple point realizations must consider corrections and uncertainties due to variations in the isotopic composition of water. Triple point cell production processes yield cells with triple point temperatures 60 µK below that implied by the definition, with a similar sized uncertainty in the value. The use of isotopically impure water for the triple point leads to a fundamental uncertainty in the definition of the kelvin of around 14 µK because of the equilibrium isotope fractionation between the ice and liquid water in the triple point cell.