coolant level going down??

[whitestoker]

hi, i just bought a 2000 f250 xlt with the 7.3 diesel.. one owner truck. guy gave me every paper of everything ever done to the truck. truck has 243k miles on it. and runs great. but i have noticed that the coolant level has gone down over a couple of days. it is not leaking it out because i dont see it on the ground. could this be a head gasket???? the temp gauge runs in the the middleish maybe a little under the middle.

a friend say it would be overheating if the head gasket was bad..
im new to the 7.3 diesel

thanks for the help guys