Thanks for that. I saw my GP today and we're starting with an eye examination. He did ask me about excessive thirst but - to be honest - I'm not sure what excessive is. I drink maybe 1L-1.5L of water a day just sitting at my desk in the office.
I will add some reasons for my guess. Your dizziness when standing up could be due to a failure of the peripheral vasculature to contract (postural syncope), but it is also characteristic of dehydration.
The excessive thirst and dehydration suggests something is causeing your body to use excess water. When insulin is not available in sufficient quantities, the body switches to an alternate method to reduce sugar levels. This alternate mechanism uses up a lot of water, and often results in dehydration.
Often, there is a sweet smell to the urine, caused by what are called ketones.
I'm sure your GP will know exactly what to do.
There is a possibility of MS, but it is more likely you have an insulin utilization syndrome. Blurred vision, excessive thirst, and sleepiness are hallmarks for this diagnosis.
You require a glucose fasting test or an insulin clamp protocol.
If these test(s) come out within normal limits, your physician can go on to explore more exotic etiologies.
I don't know for certain what is wrong with you, but a fasting test is the first step in a diagnosis.