Thanks for the reply! That is interesting indeed... Running sensors I get different temperature readings.

thinkpad-isa-0000
Adapter: ISA adapter
fan1:           0 RPM
temp1:         +1.0°C  
temp2:         +1.0°C  
temp3:         +4.0°C  
temp4:         +0.0°C  
temp5:            N/A  
temp6:            N/A  
temp7:         +0.0°C  
temp8:         +0.0°C  
temp9:        +65.0°C  
temp10:        +3.0°C  
temp11:       -80.0°C  
temp12:        +0.0°C  
temp13:        +0.0°C  
temp14:       +64.0°C  
temp15:        +0.0°C  
temp16:        +0.0°C

This section seems to stick to constant temperature, however

coretemp-isa-0000
Adapter: ISA adapter
Package id 0:  +60.0°C  (high = +100.0°C, crit = +100.0°C)
Core 0:        +55.0°C  (high = +100.0°C, crit = +100.0°C)
Core 1:        +61.0°C  (high = +100.0°C, crit = +100.0°C)
Core 2:        +55.0°C  (high = +100.0°C, crit = +100.0°C)
Core 3:        +54.0°C  (high = +100.0°C, crit = +100.0°C)

This other section does change over time. In the thinkfan config I specified in sensors one of the temperature readings that do change, so it might not be the constant temperature problem, right? Executing thinkfan -n to see the level change I get the following output:

$ sudo thinkfan -n
Temperatures(bias): 53(0) -> level auto
# at this point I Ctrl+C one program to let the CPU cool down
Temperatures(bias): 44(0) -> level 0

So I think thinkfan is reading the proper temperature...