Hyaline casts are structures formed in kidney tubules when kidney tubules secrete Tamm-Horsfall protein, according to Johns Hopkins Medicine. Hyaline casts are normal, although they indicate concentrated urine, reports Mayo Clinic.
Tamm-Horsfall protein is the most common proteins found in human urine, according to Mayo Clinic. Cells in the thick ascending limb secrete this protein. The protein aggregates and forms a gel when it is concentrated. Hyaline casts form when there is nothing else around as the gel is formed. Tamm-Horsfall protein acts as a glue that holds hyaline casts together.
Hyaline casts take on a tubular shape because they are formed in tubules, according to Johns Hopkins. Strenuous exercise may cause an increased presence of hyaline casts. Although hyaline casts do not indicate a problem, casts composed of red or white blood cells do point to kidney problems. Red blood cell casts indicate a problem with glomeruli, structures in the kidneys that filter blood and prevent the loss of important substances. White blood cell casts point to kidney inflammation.
Hyaline casts appear on a microscopic urinalysis evaluation. A microscopic urinalysis is part of a complete urinalysis, which also includes a macroanalysis and a chemical analysis. Red blood cells, white blood cells, epithelial cells, bacteria and crystals also appear as part of a microscopic urinalysis evaluation.