Both boys and girls start producing both oestrogen (female hormone) and testosterone (male hormone) in their early teen years. These hormones are responsible for the changes from child to adult. Boys produce far more testosterone, and girls far more oestrogen to create the characteristic differences between the sexes.
In the early stages of puberty, the balance between the two hormones may not be quite right. This results in irregular periods in girls, and breast bud development in boys. After a few months, the imbalance settles down, and the woman’s periods become regular, and the breast buds disappear in men. In a very small number of cases the breast development in young men may be significant enough to warrant a small operation to remove them. Boys to whom this occurs should be reassured that what is happening is completely normal, and will have no effect upon their sexual preferences or masculinity.
Male breasts sometimes enlarge in old age, with an increase in weight, and with oestrogen hormone treatment in men wanting to change sex. It may also be a symptom of a significant underlying disease including cirrhosis of the liver, cancer in the adrenal glands, overactivity or cancer of the thyroid gland, an uncommon form of lung cancer, a tumour in the testicles or a genetic condition called Klinefelter syndrome in which there is an extra sex chromosome.