A urine specific gravity test compares the density of urine with the density of water. The osmolality of the urine is inferred by measuring the urine specific gravity, which is defined as the weight of the solution compared with the weight of an equal volume of distilled water. Urine specific gravity varies with osmolality, rising by approximately 0.001 for every 35 to 40 mOsmol/kg increase in urine osmolality. However, there is a crucial difference between these measures—Urine osmolality is determined by the number of particles in the urine (e.g., urea, sodium, potassium), while the specific gravity is determined by both the number and size of the particles in the urine. This becomes important clinically when there are large molecules in the urine, such as glucose or radiocontrast media.
A specific gravity ≤1.003 is indicative of a maximally dilute urine (≤100 mOsmol/kg). This can occur from excessive water intake, diuretic use, medical problems such as diabetes insipidus, etc.
Low specific gravity suggests that urine is too diluted. The person may be drinking too much fluid or have a condition that makes them thirsty thus increasing water intake.
Low urine specific gravity has many causes, including:
Diabetes insipidus
Kidney damage or kidney failure
Drinking too much fluid, e.g., due to excess thirst (a/k/a polydipsia)
Diuretics, which cause the body to remove extra sodium from the urine
Congestive heart failure
Glucocorticoid use
(Some) anticonvulsant use
Excessive thyroid hormone supplementation or hypothyroidism, of which Hashimoto’s thyroiditis is the most common (medical studies conflict on this point)
Low-protein diet