Polyuria is the term for an excessive urine output marked by a high urine volume and frequent urination. An output of 3 litres (approximately 100 fluid ounces) or more of urine per day is considered as polyuria. Since the bladder can only store about 300 to 500ml of urine, the need to urinate frequently may be a common feature of polyuria. However, frequent urination without an inappropriately high urine output in a 24 hour period may not be polyuria.
How does polyuria occur?
Polyuria may be due to one of two processes :
- The concentration of solutes within the urine is abnormally high and this draws more fluid from the blood via osmosis.
- The concentration of solutes within the plasma is abnormally low and more water has to be passed out with urine.
Damage to the the kidney’s filtration structures (glomerulus) could also result in polyuria as the kidneys are not able to draw out excess water in the urine.
Causes of Polyuria
Polyuria is often induced by drinking excessive amounts of water and other fluids or consuming diuretics like tea, coffee, cola, or alcohol. Psychogenic polydipsia (primary) is the the perception of extreme thirst with the excessive intake of fluids and this leads to polyuria.
At other times it may due to medication (iatrogenic), particularly caused by diuretic drugs, which function by causing the body to pass out more fluid through urine in the treatment of hypertension (high blood pressure) or edema. Patients on intravenous (IV) drips may be administered excess fluids or isotonic/hypertonic saline infusions leading to polyuria.
It must first be differentiated whether the excessive urination is due to increased consumption of fluids. In this instance, drinking excessive amounts of water will read to polyuria and the use of caffeinated drinks or alcohol will further aggravate the problem as these substances act as diuretics. Certain environmental conditions, like cold weather, may result in urinary frequency, however, the quantity of urine passed in a day is usually within the normal range.
- Diabetes mellitus
- This is the most common cause of excessive urination.
- Diabetes mellitus is a result of a low levels of insulin or insulin resistance resulting in elevated blood glucose levels (high sugar levels).
- Diabetes insipidus
- Excessive thirst and urination caused by a deficiency or lack of ADH or the kidney’s resistance to ADH.
- There are different types of diabetes insipidus which are due to a number of different causes.
- Excessive Thirst
- Increased thirst (polydipsia) may be physiological, where the thirst center in the hypothalamus is affected, or psychogenic like in anxiety, stress and psychiatric illness.
- Also refer to Causes of a Dry Mouth.
- Diuretics act by increasing urine output and this is a desired effect.
- Side effects of other drugs like lithium, certain antibiotics (e.g. tetracyclines), certain antifungals (e.g. amphotericin B), large quantities of supplements like vitamin D or calcium.
- Abnormal levels of a number of electrolytes may result in excessive urination. The cause of these very high or low levels may be due to a number of causes.
- Hypercalcemia – high levels of calcium in the blood.
- Hypokalemia – low levels of potassium in the blood.
- IV Saline Drip
- Excess administration of saline
- Hypertonic saline
- Other causes
- Cushing’s syndrome
- Tube feedings
- Removal of urinary tract obstruction