Components of biological variation, including seasonality, in blood concentrations of TSH, TT3, FT4, PRL, cortisol and testosterone in healthy volunteers

OBJECTIVEThere are few detailed studies of annual or seasonal variations in hormone concentrations in man. This study examines the components of biological variation, including seasonality, in plasma TSH, total T3 (TT3), free T4 (FT4), PRL, cortisol and testosterone in healthy volunteers. DESIGNMonthly blood samplings for the assay of the above hormones were collected during one calendar year. SUBJECTSThirteen normal men and 13 normal women participated in the present study (mean age 38.7 ± 13.4 years). MEASUREMENTSAssays of TSH, TT3 and FT4 were carried out by means of Immunoradiometric assays (IRMA), PRL by ELISA, cortisol by a fluorescence immunoassay, and testosterone with RIA. The time series were analysed by means of (bivariate or multivariate) spectral and cosinor analyses. RESULTSSignificant annual, four‐monthly and biannual rhythms were detected in serum TSH; the lowest TSH values were observed in spring. A significant annual rhythm was detected in TT3, with lower values in spring and summer than in the other seasons. The peak–trough differences in the yearly variation expressed as a percentage of the mean were 29.1% and 8.2% for TSH and TT3, respectively. The yearly variation in plasma cortisol was significantly different between men and women: in men, 5.9% of the variation was explained by an annual rhythm, while in women 14.7% was explained by the fourth and seventh harmonical wave. The peak–trough differences in the yearly variation in plasma cortisol were 17.6% and 31.8% in men and women, respectively. There were no significant seasonal rhythms in PRL, FT4 or testosterone. The intraindividual/interindividual CV values were: TSH 29.3/48.4%, TT3 9.4/18.5%, FT4 7.1/9.1%, PRL 39.2/65.0%, cortisol 21.7/46.2%, and testosterone 12.6/40.8%. CONCLUSIONSThe degree of individuality measured in the plasma hormones is such that conventional population‐based reference ranges may not correctly identify major alterations in these hormones in individual subjects.