The thyroid is a butterfly-shaped gland in the lower front of the neck that is incredibly crucial in maintaining optimal function in the body. All cells, muscles, and organs in the body depend on thyroid hormone to function at their best. The thyroid produces hormones that regulate functions such as:

Metabolism
Heart function
Digestion
Muscle control
Bone maintenance
Mood
Brain development

Hypothyroidism is a condition in which the thyroid gland is not releasing enough thyroid hormone for optimal metabolic function, while hyperthyroidism is when the thyroid gland is producing too much thyroid hormone. These conditions may also have an underlying autoimmune disease. Hashimoto’s, in which the immune system attacks the thyroid gland, causing it to underproduce thyroid hormone, is often a cause of hypothyroidism. Graves’ disease is another autoimmune condition in which the immune system attacks the thyroid gland, but it instead causes it to overproduce thyroid hormone.

Both hypothyroidism and hyperthyroidism can greatly affect menstruation, fertility, maternal health, and fetal health.

Thyroid effects on menstruation

As you know, menstruation is a normal part of a woman’s monthly cycle. With each month, a woman’s body prepares for pregnancy. If pregnancy does not occur, the uterus lining is shed and blood and tissue from the uterus leave the body.

Many women with a thyroid condition find that their menstrual cycle is greatly affected by their condition, which can then affect the ability to become pregnant.

Hypothyroidism effects on menstruation
Having hypothyroidism can have a negative impact on menstruation in many ways. We discuss some of these ways below.

General hormonal changes with hypothyroidism

When thyroid hormone levels decrease, as is the case in hypothyroidism, the hypothalamus responds by making more thyrotropin releasing hormone (TRH) and the pituitary gland makes more of two hormones - thyroid stimulating hormone (TSH) and prolactin (1).

Due to the high prolactin levels, the hypothalamus’ production of gonadotropin-releasing hormone (GnRH) is suppressed. GnRH is needed to produce both follicle-stimulating hormone and luteinizing hormone in the pituitary gland. Both follicle-stimulating hormone and luteinizing hormone are important for optimal ovarian function. When the body has decreased amounts of these hormones, menstrual issues can occur (1).

Low thyroid hormone levels cause an increase in prolactin levels.

High prolactin levels can also interfere with the production of estrogen and progesterone, which can change or stop ovulation, leading to irregular or missed periods. However, women can also have high prolactin levels without having any symptoms (2). Importantly, decreased progesterone levels may also lead to an inability to maintain a pregnancy and carry a baby to term.

Oocyte maturation and FSH

With hypothyroidism, oocyte maturation may be affected (3). Oocytes are cells in the ovary that should divide to form an ovum, which will then become an embryo after fertilization.

A deficiency in thyroxine also leads to lower levels of FSH (follicle stimulating hormone) and LH (luteinizing hormone). These hormones are extremely important to the development and function of a woman’s ovaries as well as the growth of ovarian follicles before the release of an egg at ovulation (4). With insufficient levels of FSH and LH, follicles do not grow and cysts can develop, turning into polycystic ovary syndrome (PCOS). This can cause major problems with ovulation and may prevent the process completely.

Shorter second half of menstrual cycle

Hypothyroidism can also lead to lower levels of LH (luteinizing hormone) in the blood. LH is important for regulating the menstrual cycle and ovulation. With lowered LH comes a shortened luteal phase, which is the second half of the female menstrual cycle. This phase is when the egg travels down the fallopian tube to be fertilized when it comes in contact with sperm. Higher TSH levels that come with hypothyroidism can cause lower LH which may result in a short luteal phase, failure to sustain a fertilized egg (5), as well as early pregnancy loss (6). Additionally, there is an association with infertility and women who have elevated TSH levels, even if they are in the “normal” range (7).

Irregular periods

Women with hypothyroidism may have periods that are heavy, light, irregular, or even completely absent. Several things can happen that cause periods to be irregular:

Uncontrolled hypothyroidism can cause anovulation, which is a condition where the ovaries don’t release an egg during a menstrual cycle. This can lead to unusual growth and shedding of the uterine lining, which can result in heavy, unpredictable bleeding.
With the high prolactin levels, the uterine lining can become very thin and periods can stop completely.
Due to high prolactin levels, the way the pituitary gland controls the ovaries is altered, which can result in periods stopping completely.
Hypothyroidism can alter coagulation factors, which may result in heavy bleeding. (8)

While not all women with hypothyroidism will have irregular menstrual cycles, there is a much higher chance of irregular menstrual cycles for women who have hypothyroidism.

In one particular study with 171 patients with hypothyroidism, 23.4% had irregular menstrual cycles (9). In another study, researchers examined 100 participants- 50 patients who had menstrual complaints and 50 patients who had regular cycles. Of the 50 patients who had menstrual complaints, 44% had thyroid disorders. The 44% with menstrual complaints and thyroid disorders consisted of women with hypothyroidism, hyperthyroidism, and subclinical hypothyroidism (10).

Hyperthyroidism effects on menstruation

In hyperthyroidism, amenorrhea, oligomenorrhea, and anovulatory cycles are all possible. Amenorrhea is the complete absence of a menstrual cycle. Oligomenorrhea is when there are infrequent menstrual cycles- fewer than 6 to 8 cycles per year. As noted above, an anovulatory cycle is a menstrual cycle without ovulation, which means that no egg is released by the ovary during that cycle. As you can see, these are all major disturbances in the female menstrual cycle that can greatly affect one’s ability to become pregnant (11).

Effects on Fertility
The above effects on menstruation have a direct impact on fertility. We discuss some additional fertility issues below.

Implantation issues

Thyroid antibodies may lower the chances of implantation by making it quite difficult for the endometrium to receive and nurture a fertilized egg. Thyroid antibodies can make the endometrium a non receptive environment for a fertilized egg in a woman with Hashimoto’s as compared to a woman with a healthy thyroid. Changes in thyroid hormones can directly interfere with embryo attachment and implantation (12).

Decreased success of IVF

Thyroid conditions can also affect the success of IVF. In a study looking at 240 women undergoing IVF treatment, women with hypothyroidism had significantly less implantation, clinical pregnancies, and live births than women with a healthy thyroid, even when the women with hypothyroidism were treated with levothyroxine for 3 months prior to maintain TSH levels (13). This highlights the significance of thoroughly treating a thyroid condition to maintain optimal hormone levels and reproductive function.

Effects of antibodies on ovaries
Thyroid antibodies also have a direct effect on ovaries. Premature ovarian failure (POF) is when the ovaries stop working prior to the age of 40. POF is associated with autoimmune conditions about 20-30% of the time. One of the most common autoimmune causes of POF is thyroid autoimmunity (14).

Additionally, in conditions like polycystic ovary syndrome (PCOS), antibodies from Hashimoto’s thyroiditis can significantly worsen the condition, wreaking further havoc on the ovaries (15). Having Hashimoto’s thyroiditis is three times more common in women who have PCOS than women who do not have the condition (16). In a Taiwanese study, women who had Hashimoto’s also had a 2.37 times higher risk of PCOS than women without Hashimoto’s (17). Since both conditions increase infertility, having both Hashimoto’s and PCOS is highly problematic.

Effects on Maternal Health

In the event of pregnancy, having a thyroid condition may also have serious implications on maternal health. Subclinical hypothyroidism is fairly common in pregnancy, affecting up to 15% of pregnant women. Overt hypothyroidism is much more rare, occurring in about 0.2% of pregnancies. This is likely due to decreases in fertility associated with hypothyroidism as well as anovulation (18). In the case of overt hypothyroidism, there have been cases associated with an early-onset of preeclampsia-like syndrome (19, 20) or preeclampsia itself (21).

Additionally, hypothyroidism is associated with an increased risk of placental abruption, gestational hypertension, and gestational diabetes (21). Women with hypothyroidism are also at a greater risk of postpartum hemorrhage, which is heavy bleeding after giving birth. This is a serious condition, though rare. However, it happens far more frequently in women who have hypothyroidism (22).

For hyperthyroidism, some research has also found that women with hyperthyroidism are at a higher risk of developing gestational hypertension. Another condition to look out for with uncontrolled hyperthyroidism is thyroid storm. Thyroid storm is a severe and life-threatening condition that can occur with hyperthyroidism, in which extremely high levels of thyroid hormone cause diarrhea, dehydration, rapid/irregular heartbeat, high fever, shock, and death when left untreated (23).

Effects on Fetal Health

Thyroid conditions can also pose problems to the baby during pregnancy, both with hyper- and hypothyroidism. In a Danish study that analyzed 1,638,338 live births from 1978-2006, both hyperthyroidism and hypothyroidism were associated with preterm births. Additionally, babies born to mothers who had hyperthyroidism tended to be of low birth weight, while babies born to mothers who had hypothyroidism tended to be of high birth weight (24). However, other research points to lower birth weight in mothers with hypothyroidism (25).

Hypothyroidism also increases the risk of miscarriage as well as perinatal mortality (26), which refers to infant stillbirths and deaths within the first week of life. Even with a successful live birth, hypothyroidism can still have an impact, because the baby relies on the mother for thyroid hormone during development. Some research has shown that babies of mothers with hypothyroidism can have lowered IQ and impaired psychomotor development (27).

In babies of mothers who have Graves’ disease, 1% of these babies may also have Graves’. These babies may need hospitalization.

While checking thyroid status isn’t the standard care in pregnancy, we argue that it should be. Additionally, it’s important to do a thorough check of the thyroid to make sure it’s functioning at optimal levels.

If you suspect that a thyroid condition might be affecting your cycles, your ability to conceive, or you want to make sure your thyroid is supporting and not hurting your pregnancy, please contact us. We specialize in supporting those with thyroid disorders and would love to help.

References:

Effects of Thyroid Disease on Menstruation, Fertility, and Mother & Baby's Health

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Effects of Thyroid Disease on Menstruation, Fertility, and Mother & Baby's Health

The Effects of Male Thyroid Conditions on fertility

SIBO: A common digestive disorder that is often missed

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