Chapter 1 14 through the so-called neonatal minipuberty, showing increases in LH, FSH and testosterone in the first six months of life (Kuiri-Hänninen et al., 2014). Studies also show that this postnatal minipuberty can affect gonadal and neuronal development in male babies (Hines et al., 2016). Following this phase, testosterone levels are also low during childhood and increase during puberty in boys, stimulating the development of masculine secondary sex characteristics such as beard growth, low voice and male musculature. Although reproductive-age males do not show the pronounced hormone fluctuations that is seen in females, testosterone levels in adult males do fluctuate in a 24-hour rhythm: they are highest early in the morning, and decline during the daytime. During sleep at night, especially deep sleep, testosterone production is highest, resulting in high testosterone levels in the morning again. There is no marked point at which testosterone production strongly declines in men: some studies show that testosterone levels decrease with age (Feldman et al., 2002; Harman et al., 2001), but it is not yet clear whether this directly caused by ageing or indirectly via increased comorbidity and lifestyle factors, especially obesity (Dean et al., 2015; Tajar et al., 2012). Some men show very low testosterone levels, which is also called hypogonadism, which can be treated with testosterone supplementation. Testosterone levels in women are lower than in men. Testosterone in women also shows small circadian fluctuations, with higher levels in testosterone in the morning (Bungum et al., 2013), as well as fluctuations throughout the menstrual cycle, when it peaks around ovulation and remains higher in the luteal phase (Rothman et al., 2011). Throughout the lifespan, testosterone levels also decline in women from the age of 45, independently from the menopausal transition (Burger et al., 2000; Davison et al., 2005). Although excessively high testosterone levels in women are be associated with hormonal disorders, most commonly with polycystic ovarian syndrome (Rodriguez Paris & Bertoldo, 2019), the clinical significance of testosterone in women is not so clear as it is in men, and testosterone supplementation in women is not part of regular clinical care.
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