Regular and predictable menstrual cycles in women occur if the ovarian hormones oestrogen and progesterone are secreted in an orderly fashion in response to stimulation by the hypothalamus and pituitary. A normal menstrual cycle occurs between 21-35 days with 3-10 days of bleeding (menstruation) and 30-150 ml of blood loss with usually no more than 2 heavy days. Around the 14th day, ovulation occurs.
During a normal ovulatory cycle, the hypothalamus in the brain produces a chemical called gonadotrophin-releasing hormone (GnRH), which stimulates the pituitary gland to release follicle-stimulating hormone (FSH). This FSH causes an ovarian follicle to grow. The developing follicle produces oestrogen, which stimulates growth of the endometrium (lining of the uterus). In mid cycle, a surge of luteinizing hormone (LH) occurs along with a FSH surge from the pituitary gland resulting in ovulation i.e. release of egg. After the ovum is released, FSH and LH levels fall. At the site of the ruptured follicle, a corpus luteum develops, which secretes progesterone. This hormone prepares the endometrium for implantation (of a fertilised egg) and maintenance of a pregnancy. If pregnancy does not occur, the corpus luteum involutes causing a rapid decline in the levels of oestrogen and progesterone. This leads the secretory endometrium to break down and be shed during the ensuing menstrual period.
In women, LH stimulates oestrogen and progesterone production from the ovary. In men, LH stimulates testosterone production from the interstitial cells of the testes (Leydig cells).
Progesterone acts to stabilize the tissue lining of the uterus (endometrium) so if it is absent, such as with ovarian anovulation, irregular and heavy menstrual bleeding often occurs after a period without any menstrual bleeding. It tends to promote vaginal dryness by counteracting the effect on lubrication of estrogens and it can also decrease the amount of menstruation or block it entirely by reversing oestrogen effect on the growth of the uterine lining.
The thyroid gland produces hormones called T4 and T3 when stimulated by a hormone called thyroid stimulating hormone (TSH) secreted by the pituitary gland. The secretion of TSH, in turn, is regulated by the hypothalamus gland which secretes TSH Releasing Hormone (TSRH). Thyroid function tests usually involve several tests, which help to diagnose different thyroid diseases. These tests may include the blood (serum) levels of thyroid hormones T3 & T4, measurement of the pituitary production of TSH, iodine uptake scan, thyroid antibodies, level of thyroid binding globulin, a thyroid ultrasound etc.
You need to talk to your doctor who will be able to explain what tests are proposed and why are they being done.