Heat pipe performance degrading
The main purpose of heat pipe is to TRANSFER heat from hot place (CPU) to cold place (heat exchanger/fins). To remove the heat from "cold end", the end still must maintain some significant delta over ambient air. First indication that a heat pipe has lost its functionality would be if the CPU end is overheated (as measured either by internal sensor, or by touch of a thermocouple), while the cooled end is at room temperature. 

And yes, the pipes are factory sealed to maintain low pressure, and quality of seal/weld can deteriorate over time.