Polyuria (excessive urination) and polydipsia (excessive drinking) occur in three out of ten horses that suffer from PPID and insulin resistance/EMS. It is therefore an important clinical sign. Let’s have a look at how these problems arise.
One of the consequences of insulin resistance is an increased level of glucose in the blood (hyperglycemia). Normally the kidneys are able to remove glucose from the pre-urine before it becomes urine. However, if the amount of glucose in the pre-urine is higher than the maximum absorption capacity of the kidneys (the renal threshold), glucose will end up in the urine. This phenomenon is called glycosuria. The glucose retains water, causing the horse to urinate more (polyuria) and as a result, drink more (polydipsia).
The increased cortisol level that we can (but certainly not always will) find in the blood of horses with PPID also has an inhibitory effect on the antidiuretic hormone (ADH). By stimulating water resorption by the kidneys, ADH ensures that less water ends up in the urine. An inhibition of ADH is therefore the second cause of polyuria in horses with PPID. Moreover, less ADH is produced. This is because the enlarged intermediate lobe of the pituitary gland presses on the part of the posterior lobe (pars posterior, neurohypophysis) where ADH is stored and from which it enters the bloodstream.