Hi, I have lost a lot of weight from

Patient: Hi, I have lost a lot of weight from (87kilos ) to (74kilos) withing two years.I am not sick but always thirsty and and drink about 6 to 7 litres of water every day. I have my thighs and calf muscles ache a lot. My friends think that was discharged from the Hospital by looking at my face. Please help me.

Doctor: