Hair growth is regulated by hormonal changes in the body. These hormones can affect not only how much grows, but whether growth stops and no new hair grows. The processes that regulate how much does or doesn't grow, and what type of hair grows, can go awry and cause abnormal hair growth, as well.
Gonadotropins are pituitary gland hormones that affect the gonads, or testes, and ovaries. The two gonadotropin hormones are called the luteinizing hormone and the follicle-stimulating hormone. These help the testes and ovaries produce male and female sex hormones, as well as eggs and sperm--the follicle in this case isn't hair, but ovarian follicles that function as protective sacs for eggs. Columbia University's Health Services department notes in its Go Ask Alice information database that neither of these hormones really have anything to do with hair growth directly. However, not only do they influence the production of sex hormones that influence hair growth, they can interact with the progesterone, a female sex hormone, in the Depo-Provera birth control shot in women and lead to hair falling out and not regrowing. Columbia University notes that hair should begin growing again once the shot is discontinued.
The hormones with the most direct effect on hair growth are androgens, the male sex hormones that include testosterone. Both men and women produce androgens--a gender just produces more of one and less of the other. Facial and body hair are considered secondary sex characteristics that form in response to androgens. Sensitivity to and overproduction of androgens in women can lead to hirsutism, or excess body and facial hair. Sometimes the extra effect is due to a condition such as polycystic ovarian syndrome, in which excess androgen production is a symptom, and in other cases no specific cause is apparent. Androgens also contribute to male- and female-pattern baldness, somehow preventing hair follicles from transitioning out of their normal resting phase. The University of Maryland Medical Center notes the reason for this is unclear.
Hormones don't have to have a direct connection to hair to influence its growth or loss. Such is the case with thyroid hormones. The hypothalamus, a section of your brain, produces thyroid-releasing hormone, which pairs up with the pituitary gland to produce thyroid-stimulating hormone. This in turn acts on the thyroid to produce thyroid hormones, which travel throughout the body. A lack of thyroid hormones, caused by either iodine deficiency or thyroid disease, leads to hypothyroidism and possible hair loss. The mechanism behind this is not well-explained, although a 1998 Japanese study in Biochemical and Biophysical Research Communications found thyroid hormone "takes part in an inhibitory regulation of ovarian hormonal secretion" in rats.