Delayed puberty is defined as the absence of secondary puberty or signs of puberty by age 13 in girls and age 14 in boys. The first sign of puberty in girls is breast development. This happens as the ovaries become active and produce estrogen. In boys, one of the earliest signs of puberty is testicular growth caused by increased production of follicle-stimulating hormone from the pituitary gland. Delayed puberty is diagnosed when there is no breast development in girls by age 13 or testicular growth in boys by age 14. In over 90% of cases, delayed puberty is due to what is known as constitutional retardation of growth and puberty. This means that it occurs in children who are healthy but simply have a slower than average rate of physical development. These children are usually short compared to others of the same age and are often thin and have a family history of delayed puberty. Sometimes delay in puberty and growth can be due to chronic illness, malnutrition, excessive exercise and even stress. For example, diabetes, cystic fibrosis and celiac disease can delay puberty, while excessive exercise and anorexia, where the proportion of body fat is very low, can also delay puberty. Rare causes of delayed or absent puberty may be caused by loss of hormonal secretions from the hypothalamus and pituitary gland or by disorders of the testes or ovaries. Puberty begins with increased production of luteinizing and follicle-stimulating hormone from the pituitary gland, which is driven by increased release of gonadotropin-releasing hormone from the hypothalamus. When something goes wrong with this control, then hypogonadotropic delayed puberty occurs. Testicular or ovarian disorders can cause hypergonadotropic delayed puberty. In this case, the levels of gonadotropins are high, but the ability of the testicles or ovaries to detect this is impaired and therefore they cannot respond to the gonadotropins by producing testosterone and estrogens respectively. Chromosomal disorders such as Klinefelter syndrome in boys and Turner syndrome in girls result in abnormal development of the testes and ovaries with reduced or absent production of testosterone or estrogen. Other gonadal disorders include failure of the testicles to descend (cryptorchidism), testicular torsion, pelvic/abdominal radiation, gonadal surgery, and mumps (although mumps rarely causes testicular damage before puberty). Delayed puberty occurs in about 3% of children, and 90% of these cases are caused by constitutional delay. Constitutional delay is 10 times more common in boys than in girls. References: https://www.yourhormones.info/endocrine-conditions/delayed-puberty/