The effect of altitude on thrust output is a function of density. The higher you go the less air pressure there is and the colder it gets. As pressure decreases thrust decreases, but as temperature decreases thrust increases. However, the pressure drops off faster than the temperature so that there is actually a drop off in thrust with altitude.
At about 36,000ft (tropopause) the temperature stops falling and remains constant while the pressure continues to fall. As a result, the thrust will drop off more rapidly. This makes 36,000ft the optimum altitude for long range cruising, just before the rate of thrust fall off increase. Lighter weights can offset thrust lapse rate and retain optimum conditions at higher flight levels as shown in cruise performance data.