It is well known that the hormones produced by the thyroid gland are related to various aspects of human reproduction. The thyroid gland affects the metabolism of sex hormones and ovarian function in women, which is often expressed as menstrual irregularities. Thus, menstrual disorders are 2.5 times more frequent in women with hyperthyroidism than in the normal population.
What is the thyroid gland, which is hyperthyroidism and how does it affect menstruation?
The thyroid gland is a small bowel-shaped gland or butterfly located on the neck, on both sides of the trachea, below the walnut. This gland secretes the thyroid hormone T4, with multiple functions in the regulation of metabolism and growth.
The function of the thyroid gland is regulated by an area of the brain, the hypothalamus, which also regulates all reproductive functions. The hypothalamus secretes thyrotropin-releasing hormone (TRH) that reaches the pituitary gland and causes it to secrete thyroid stimulating hormone (TSH). TSH reaches the thyroid gland through the blood and stimulates the secretion of thyroid hormones (T3 and T4). When these reach the appropriate levels in blood, the hypothalamus stops producing HRT.
It happens that the same hypothalamus regulates the ovarian functions, by means of a mechanism similar to the one used for the thyroid. Segregates GnRH that stimulates the secretion in the pituitary of the hormone FSH or folliculo stimulating. FSH reaches the ovaries through the blood by stimulating the growth of ovarian follicles and the maturation of oocytes and the synthesis of estrogens. During the second half of the cycle, GnRH causes secretion in the pituitary of other hormones, LH or luteinizing. The LH reaches the ovary and causes ovulation, the formation of the corpus luteum and the secretion of progesterone.
When the LH stimulus is over, menstruation occurs. Therefore, the thyroid gland and ovaries are governed from the same regulatory center.
Hyperthyroidism is characterized by the suppression of the secretion of hormone TRH in the hypothalamus to the consuencia of very high levels of T3 and T4 in blood. These elevated levels of T3 and T4 alter the functioning of the hypothalamus, which ends up impacting the function of the ovaries. LH secretion is altered by GnRH, leading to changes in the menstrual pattern. LH levels in both the follicular and luteal phases are also significantly higher in women with hyperthyroidism than in normal women.
Hyperthyroidism results in increased levels of a protein in blood, the sex hormone carrier globulin (SHBG). As a result, plasma estrogen levels may be double or three times higher in women with hyperthyroidism during all phases of the menstrual cycle. This happens because estrogens remain bound to this protein which prevents them from being eliminated from the body.
In addition, thyroid hormones are present in the follicular fluid of the ovarian follicles, so it is believed that there is a direct action of these hormones on the ovaries that has effects on the menstrual cycle.
What are the different alterations of the menstrual cycle?
There is much confusion among physicians about the definition of the different terms used to characterize menstrual disorders:
1. Alterations of the duration of the menstrual cycle:
Oligomenorrhea: interval between two periods of more than 35 days
Polymenorrhea: interval between two periods of less than 21 days,
Amenorrhea: missed menstruation for more than 3 months
2. Alterations in the amount of menstrual flow.
Hypomenorrhea: decrease of more than 20% in menstrual flow,
Hypermenorrhea: increase of more than 20% in menstrual flow compared to previous periods,
Menorrhagia: heavy menstrual bleeding
What is the frequency and type of menstrual cycle abnormalities in women with hyperthyroidism?
Hyperthyroidism often causes menstruations of less volume (hypomenorrhea), while heavy menses are more frequent in hypothyroidism.
The frequency of menstruation is also related to thyroid disease. Hyperthyroidism can often cause uncommon periods (oligomenorrhea and occasionally stop menstruation altogether (amenorrhea). Hypothyroidism, on the other hand, can cause more frequent and longer menstrual periods. Smoking aggravates the development of menstrual disorders in women with hyperthyroidism.
When to go to the doctor?
- If the menstruation lasts more than seven days,
- If menstruation is more painful than normal,
- If menstruation occurs less than 21 days apart,
- If Menstruation stops for more than three months.