Administration measurement of urinary melatonin: A useful tool for monitoring serum melatonin after its oral administration

The relevance of measuring urinary melatonin (MLT) for human pineal researc h is sometimes questioned, and the relationship among serum levels of MLT, urinary excretion of the unmetabolized hormone, and excretion of MLT's main metabolite, 6-hydroxymelatonin sulfate (aMT6s), is still uncertain. We applied a well established RIA for measuring MLT in serum to urine sampl es. characterized its criteria of performance in this body fluid, and used it for human studies. In 16 adolescents, the endogenous overnight MLT secre tion, expressed as the area under the concentrat ion time curve, correlated significantly with the amounts of urinary aMT6s (r = 0.86; P < 0.0001) and urinary MLT (r = 0.70; P 0.0027) excreted during a 16-h observation period . Oral administration of 3 mg exogenous MLT in 17 healthy volunteers result ed in peak MLT serum levels differing 28-fold among subjects (940-27,240 pg / mt; range). In this study urinary MLT, but not aMT6s, excretion was assoc iated with blood MLT concentrations (r = 0.76; P = 0.0004 vs. r = 0.02; P = 0.93, respectively). Thus, endogenous MLT production can be assessed accur ately by measuring either aMT6s or MLT excretion. After oral application of MLT, however, only measurement of MLT excretion is a reliable marker of se rum concentrations. Determination of MLT in urine may prove to be a useful tool for drug monitoring after oral administration of the pineal hormone.