Background: The hypothalamic-pituitary-gonadal axis is transiently activated during the postnatal months in boys, a phenomenon termed “minipuberty” of infancy, when serum testosterone (T) increases to pubertal levels. Despite high circulating T there are no signs of virilization. We hypothesize that free T as measured in saliva is low, which would explain the absence of virilization. Methods: We measured serum total T and free T in saliva using liquid chromatography-tandem mass spectrometry (LC-MS/MS) in 30 infant boys, aged 1–6 months, and in 12 adolescents, aged 11–17 years. Results: Total serum T in all infants was, as expected, high (172 ± 78 ng/dL) while salivary T was low (7.7 ± 4 pg/mL or 0.45 ± 0.20%). In contrast, salivary T in the adolescents was much higher (41 ± 18 pg/mL or 1.3 ± 0.36%) in relation to their total serum T (323 ± 117 ng/dL). We provide for the first time reference data for salivary T in infants. Conclusion: Measurement of salivary T by LC-MS/MS is a promising noninvasive technique to reflect free T in infants. The low free T explains the absence of virilization. The minipuberty of infancy is more likely of intragonadal than peripheral significance.

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