Hello,
If the ins and outs don't match completely on any given day, I wouldn't necessarily conclude that the kidney is not behaving properly. Do you account for the volume of water contained in foods (especially fruits, vegetables etc)? If the kidney was inappropriately producing more urine than it should - a condition referred to as polyuria - then this would be evaluated by measuring the quantity and the composition of the urine to determine if the kidney is mal-absorbing predominantly solutes (salts, glucose) or water. To determine this would require speaking with your physician to have the appropriate tests arranged. I would not think that polyuria from a kidney problem would come and go as you describe, however, Furthermore, it would be accompanied by symptoms consistent with low blood pressure (e.g as dizziness) or water deficiency (e.g thirst) as well as electrolyte disturbance and/or kidney function impairment if present.
Feel free to post any additional follow-up for more comment.
Dr. Jordan Weinstein