Test results should, therefore, always be interpreted using the reference range from the laboratory that produced the result. Using the molecular weight of 362.460 g/mole, the conversion factor from µg/dL to nmol/L is approximately 27.6; thus, 10 µg/dL is about 276 nmol/L. Cortisol follows a circadian rhythm, and to accurately measure cortisol levels is best to test four times per day through saliva. An individual may have normal total cortisol but have a lower than normal level during a certain period of the day and a higher than normal level during a different period. Therefore, some scholars question the clinical utility of cortisol measurement.Cortisol is lipophilic, and is transported bound to transcortin (also known as corticosteroid-binding globulin) and albumin, while only a small part of the total serum cortisol is unbound and has biological activity. Serum cortisol assays measures total cortisol, and its results may be misleading for patients with altered serum protein concentrations. The salivary cortisol test avoids this problem because only free cortisol can pass through the salivary barrier. Transcortin particles are too large to pass through this barrier.Automated immunoassays lack specificity and show significant cross-reactivity due to interactions with structural analogs of cortisol, and show differences between assays. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) can improve specificity and sensitivity.
Stress and mood
Sustained stress can lead to high levels of circulating cortisol (regarded as one of the more important of the several "stress hormones"). Such levels may result in an allostatic load, which can lead to various physical modifications in the bodys regulatory networks. Effects during pregnancy
During human pregnancy, increased fetal production of cortisol between weeks 30 and 32 initiates production of fetal lung pulmonary surfactant to promote maturation of the lungs. In fetal lambs, glucocorticoids (principally cortisol) increase after about day 130, with lung surfactant increasing greatly, in response, by about day 135, and although lamb fetal cortisol is mostly of maternal origin during the first 122 days, 88% or more is of fetal origin by day 136 of gestation. Although the timing of fetal cortisol concentration elevation in sheep may vary somewhat, it averages about 11.8 days before the onset of labor. In several livestock species (e.g. cattle, sheep, goats, and pigs), the surge of fetal cortisol late in gestation triggers the onset of parturition by removing the progesterone block of cervical dilation and myometrial contraction. The mechanisms yielding this effect on progesterone differ among species. In the sheep, where progesterone sufficient for maintaining pregnancy is produced by the placenta after about day 70 of gestation, the prepartum fetal cortisol surge induces placental enzymatic conversion of progesterone to estrogen. (The elevated level of estrogen stimulates prostaglandin secretion and oxytocin receptor development.) Exposure of fetuses to cortisol during gestation can have a variety of developmental outcomes, including alterations in prenatal and postnatal growth patterns.