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Clinical Chemistry 45: 316-317, 1999;
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(Clinical Chemistry. 1999;45:316-317.)
© 1999 American Association for Clinical Chemistry, Inc.

Letters

Urinary Free Cortisol Is Not Affected by Short-term Water Diuresis

Martin Fenske

Department of Animal Physiology, NWI, University of Bayreuth, 95440 Bayreuth, Germany, Fax 49-921-552794; E-mail painter{at}uni-bayreuth.de


To the Editor:

Urinary free cortisol (UFC) has been shown to be a reliable measure of adrenocortical secretion, and it is generally accepted as being an index of the free fraction in the plasma. However, the interpretation of UFC results reportedly can be compromised when the urine volume in patients is considerably increased because a high fluid intake (5 L/day) increases UFC in healthy subjects (1) and UFC was reported to be closely related with the changes in urine volume in women (2). The aim of this work was to determine whether short-term changes in urine volume influence UFC.

I studied 15 volunteers (6 women and 9 men) with normal body mass indexes (23.9 ± 1.9 kg/m2) and ages from 23 to 52 years. Informed consent was obtained from all volunteers and our institution's responsible committee. Subjects were asked to empty their bladders at 0900 and to collect urine samples at 30-min intervals. At 1000, 10 of the volunteers (6 women and 4 men) were asked to drink 1 L of water within 5 min. The volume of each urine sample was measured and adjusted to a final volume of 300 mL. Urine samples with a volume >300 mL were not diluted. Creatinine and cortisol were measured by a colorimetric method (3) or by RIA (4). The results are presented as mean ± SE. Statistical comparisons were made using the Mann–Whitney U-test for unpaired data.

As expected, the urine volume was significantly greater during water diuresis (P <0.002, 60–180 min after water ingestion) than during the control period (0–180 min, no water ingested). In contrast, neither urinary excretion of creatinine nor of UFC changed significantly (Fig. 1 ). A similar lack of change of UFC was found when cortisol concentrations in 40 of the samples were measured again by a combined thin-layer chromatography/RIA method (5). Similar to the results summarized in Fig. 1 , UFC values of controls and volunteers ingesting 1 L of water did not differ significantly. However, in all 40 samples, UFC amounts measured by the latter method were significantly lower than values of the same nonchromatographed samples [see also Ref. (6)].



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  		Figure 1. Influence of short-term water diuresis on the excretion of urine, creatinine, and UFC.

Eight urine samples were collected in 30-min intervals. After collection of the second sample (0 min), 10 of the 15 volunteers were asked to ingest 1 L of water within 5 min. Values are given as mean ± SE, n = 5 (open columns, no water intake) or n = 10 (filled columns, ingestion of 1 L of water); *P <0.002.

The significant finding of this work is that urinary creatinine and UFC were not influenced by short-term water diuresis. These results are in contrast with previous data that showed a close relationship between UFC and changes in urine volume in children (1), women (2), or male/female volunteers (1). The reason for this discrepancy is not known at this point. The differing results may depend on the varying fluid volumes ingested, the fact that urine was collected for 3 or 24 h, and the short time period of increased urine output in this study. Irrespective of the need to study the relationship between UFC and urine volume in more detail, the results presented here show that healthy subjects with a moderate, short-term increase in urine volume are not at a greater risk of excreting increased cortisol.


References

  1. Mericq MV, Cutler GB, Jr. High fluid intake increases urine free cortisol excretion in normal subjects. J Clin Endocrinol Metab 1998;83:682-684. [Abstract/Free Full Text]
  2. Baum CK, Davison MJ, Landon J. Urinary free cortisol excretion by normal subjects. J Endocrinol 1974;63:47P-48P.
  3. Haeckel R. Simplified determination of "true" creatinine concentrations in serum and urine. J Clin Chem Clin Biochem 1980;18:385-394. [ISI][Medline] [Order article via Infotrieve]
  4. Fenske M. Dissociation of plasma and urinary steroid values after application of stressors, insulin, vasopressin, ACTH, or dexamethasone in the Mongolian gerbil. Exp Clin Endocrinol Diabetes 1996;104:441-446. [ISI][Medline] [Order article via Infotrieve]
  5. Fenske M. Determination of cortisol in guinea pig urine by high-performance thin-layer chromatography, high performance liquid chromatography, and thin-layer chromatography/radioimmunoassay. Chromatographia 1997;44:50-54.
  6. Deck KA, Eberlein L, Vetter H, Hillen H. Die Bestimmung von freiem Cortisol im Urin. Dtsch Med Wschr 1976;101:818-821. [Medline] [Order article via Infotrieve]
  7. Bertrand PV, Rudd BT, Weller PH, Day AJ. Free cortisol and creatinine in urine of healthy children. Clin Chem 1987;33:2047-2051. [Abstract/Free Full Text]



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This Article
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