Isosthenuria refers to the excretion of urine whose specific gravity (concentration) is neither greater (more concentrated) nor less (more dilute) than that of protein-free plasma, typically 1.008-1.012. Isothenuria reflects renal tubular damage/failure of renal medullary function.
A closely related term is hyposthenuria, where the urine has a relatively low specific gravity, though not necessarily equal to that of plasma. Therefore, unlike isosthenuria, this condition is not associated with renal failure as the kidney tubules have altered the glomerular filtrate
This is the hallmark of such disease states as chronic and acute renal failure in which the kidneys lack the ability to concentrate or dilute the urine and so the initial filtrate of the blood remains unchanged despite the need to conserve or excrete water based on the body's hydration status. 
- "isosthenuria" at Dorland's Medical Dictionary
- "hyposthenuria" at Dorland's Medical Dictionary
- De Mais, Daniel. ASCP Quick Compendium of Clinical Pathology, 2nd Ed. ASCP Press, Chicago, 2009.
- Gupta AK, Kirchner KA, Nicholson R, et al. Effects of alpha-thalassemia and sickle polymerization tendency on the urine-concentrating defect of individuals with sickle cell trait and increased frequency of urinary tract infections J. Clin. Invest. Vol 88, Issue 6, pgs 1963-8. Dec 1991. PMID = 1752955. PMC = 295777. DOI = 10.1172/JCI115521
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