What can be causing excessive thirst and frequent urination?

Patient: Hello. I’m a 24 year old, healthy male. For the last few weeks I have been suffering from increased thirst, parched throat and frequent urination. The doctor suspected UTI and put me on antibiotics. The problem went away but returned once I had completed the course after the next few days. I was put on antibiotics again, only for the problem to disappear and return again but in a mild form after the course was over. I have undergone a blood test for diabetes, a routine urine analysis and a urine culture. All reports have turned out to be normal. What can be causing these symptoms?

Symptoms: Frequent urination, excessive thirst