Decreased urine specific gravity means that there is less urine mass in the urine and the concentration of urine decreases, and uremia is the primary disease of decreased urine specific gravity. Urogenital disorders are characterized by polyuria, irritable thirst, decreased urine specific gravity and hypotonic urine. The decrease in specific gravity of birds caused by urogenital disorders is commonly seen in young adults, with a ratio of 2:1 between men and women, and hereditary NDI is mostly seen in children. The following is an introduction to the main causes of decreased urine specific gravity 1, When the blood osmolarity increases significantly during water fasting, and the urine specific gravity decreases, there is no release of AVP when hypertonic saline is injected, and this type does have AVP deficiency. 2, When urine osmolality suddenly increases during water fasting, but there is no osmolality threshold when saline is injected, these patients lack osmolality sensing mechanism and are able to stimulate AVP release only when severe dehydration leads to low school volume. 3, As plasma osmolality rises, urine osmolality increases slightly and the AVP release threshold rises; these patients have a slow AVP release mechanism, or reduced osmolarity receptor sensitivity.