The gonads are responsible for secreting sex hormones. The initiation of this hormonal activity is a complex, multi-phased process spanning from fetal life through infancy and culminating in adolescence. This entire process is orchestrated by the Hypothalamic-Pituitary-Gonadal (HPG) axis, a regulatory system involving the brain’s hypothalamus, the pituitary gland, and the gonads themselves. The HPG axis governs the release of these hormones for development and reproductive function.
The Initial Surge Fetal Development and Sexual Differentiation
The earliest instance of sex hormone secretion occurs during the first trimester of fetal development. Gonadal differentiation, the process where the initially indifferent gonads become either testes or ovaries, begins early in gestation, typically around the sixth to eighth week. This differentiation depends on the presence or absence of the SRY gene located on the Y chromosome.
If the SRY gene is present, it triggers the development of the testes. These fetal testes begin producing testosterone, stimulating the development of the male internal reproductive tract. Simultaneously, the testes secrete anti-Müllerian hormone (AMH), which causes the regression of the structures that would otherwise form the female reproductive system.
In a female fetus, the absence of the SRY gene allows the indifferent gonads to develop into ovaries, and the lack of testosterone and AMH leads to the development of the female reproductive tract. This early hormonal burst is not sustained and is largely driven by placental hormones and the fetal HPG axis, peaking around 20 to 24 weeks of gestation before becoming mostly inactive near term.
The Temporary Activation Mini-Puberty in Infancy
Following birth, the HPG axis experiences a temporary phase of re-activation known as mini-puberty. This brief hormonal surge occurs because the newborn is no longer exposed to the high levels of placental hormones that had suppressed the axis during late gestation. Within the first few days or weeks of life, the hypothalamus begins to release gonadotropin-releasing hormone (GnRH), stimulating the pituitary gland to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
This temporary stimulation causes the gonads to produce sex hormones. In male infants, testosterone levels typically peak between one and three months of age, contributing to penile and testicular growth. For female infants, FSH and estradiol levels also rise but often fluctuate for a longer period, sometimes remaining elevated up to two years. This activation is thought to play a role in the final maturation of the reproductive organs before the HPG axis enters a long period of quiescence until adolescence.
The Definitive Start The Hormonal Cascade of Puberty
The sustained start of sex hormone secretion occurs during puberty, beginning after the long, quiet phase of childhood. This process, known as gonadarche, is triggered by the re-activation of the HPG axis. The initial signal appears to be an increase in the pulsatile release of GnRH from the hypothalamus, driven by neuronal signals, including the peptide kisspeptin.
The pituitary gland secretes higher, more sustained levels of LH and FSH. The first hormonal change often detected is a nocturnal rise in LH secretion, which precedes the visible signs of puberty by about a year. This hormonal shift is facilitated by a decrease in the sensitivity of the hypothalamus and pituitary to the negative feedback effects of sex hormones, meaning it takes a much higher concentration of gonadal hormones to suppress the HPG axis.
The increased LH and FSH then act directly on the gonads. In males, LH stimulates the Leydig cells in the testes to produce testosterone, while FSH promotes sperm production. The sustained rise in testosterone causes the enlargement of the testes, growth of the penis, and the development of secondary sex characteristics, such as deepening of the voice and growth of pubic and facial hair.
In females, FSH stimulates the ovarian follicles, leading to the production of estrogen, which drives breast development and the growth spurt. LH stimulates the production of progesterone and contributes to the ovarian cycle. The average onset of puberty occurs between 8 and 14 years in females and 10 and 16 years in males, with the sequence of physical changes being highly predictable despite individual variation in timing. This final, sustained hormonal activation establishes full reproductive capacity.