Metabolic disease like diabetes or thyroid disorders are certainly possible. They can be tested for with various blood tests.
You can also consider any urinary tract infection - which can exacerbate urinary frequency. Sending the urine off for analysis and culture can be done to evaluate this.
I would also obtain a metabolic panel to evaluate the electrolytes and kidney function. If there are fluctuations in the sodium level, this can affect the thirst mechanism, and point to possible diseases where this is altered.
These options can be discussed with your personal physician.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
http://www.straightfromthedoc.com