The thyroid gland is a small gland located in the neck, responsible for making thyroid hormones which help regulate metabolism. Thyroid dysfunction is very common, affecting up to 10% of the population at any given time. In general there are two forms of dysfunction: decreased thyroid hormone production (hypothyroidism) and increased thyroid hormone production (hyperthyroidism). Underactive disease, or hypothyroidism, affects 2-10% of individuals, and occurs 5-8 times as often in women than men. Overactive disease, or hyperthyroidism, is less common, affecting 1-5% of individuals. With both forms of thyroid disease, women are affected far more commonly than men (5-8x more often).
The symptoms of thyroid disease are easily recognized, but are also commonly seen in other disease presentations and among healthy individuals. Hypothyroidism presents with weight gain, fatigue, cold intolerance, hair loss, constipation, muscle aches, and difficulty concentrating. Hyperthyroidism often presents with weight loss, tremors, hair loss, anxiety, irregular heart beat (palpitations), and sweating. Both underactive and overactive disease can result in menstrual irregularities in women.
Most thyroid diseases are caused by autoantibodies against components of the thyroid gland. Antibodies are molecules made by our immune system to fight infection; in the case of auto-antibodies, our immune systems mistakenly make molecules to fight a part of our body that the immune system should recognize as a normal part of the body. The end result is organ damage, which either causes decreased thyroid hormone production (hypothyroidism) or increased hormone production (hyperthyroidism).
How do we screen for thyroid disease? Physicians will generally start with a thyroid stimulating hormone level (TSH). The TSH hormone is made in the pituitary gland (in the brain) and is sent to the thyroid gland to tell it to make thyroid hormone (think of the accelerator in your car). The thyroid then makes thyroid hormones (T4 and T3) which circulate back to the pituitary gland; the pituitary gland then recognizes that there is sufficient circulating T4 and T3 and keeps the TSH in a normal range (think of a feedback loop). When there is too little thyroid hormone, the pituitary should respond by increasing the TSH level (as if stepping on the accelerator harder to make the car go faster), therefore underactive thyroid disease typically presents with an elevated TSH level. In contrast, if there is too much thyroid hormone, the TSH is usually suppressed, because the high levels of T4 and/or T3 have signaled back to the pituitary that there is no need for TSH to keep signaling for more T4 and T3 to be made (think of the accelerator being pushed as hard as possible, there would be no need for you to push harder to go faster). There are other conditions in which this relationship is not as straightforward as just described, and for this reason it is important to rely on your physician to correctly interpret hormone results.
How do we treat thyroid disease? For the two abnormalities described previously, the goal is to restore the thyroid hormone levels back to normal. For hypothyroid disease, we replete the patient with thyroid hormone. Usually after about 4 weeks of treatment the TSH level is rechecked and the dose adjusted until the TSH is in a normal range. For hyperthyroid disease, treatment options include medications to suppress thyroid production (propylthiouracil and methimazole), radioiodine ablation, and surgical removal of the thyroid. The optimal treatment can be decided with your physician. There are other causes of thyroid diseases (tumors, cancer, nodular goiter) that are treated differently and beyond the scope of this blog.
You may be asking yourself, why does a fertility specialist care about the thyroid? The answer is normal thyroid function is critical for normal menstrual function and for optimal pregnancy outcomes. Hypothyroidism has been associated with increased risks of miscarriage, pre-eclampsia (blood pressure disease in pregnancy), and low birth weight babies. Untreated hypothyroidism can also result in mental deficiencies in children, which in the most severe form is known as cretinism. Some women do not have overt hypothyroidism, but have mild lab abnormalities that we would call subclinical hypothyroidism (high TSH but normal thyroid hormone levels). Subclinical hypothyroidism has also been associated with higher rates of miscarriage.
Hyperthyroidism is also associated with adverse outcomes, including pre-term delivery, pre-eclampsia, maternal heart failure, low birth weight babies, and miscarriage. Fetal hyperthyroidism can also occur as a result of maternal autoantibodies passing through the placenta, and causing fetal goiter (enlarged thyroid) which in severe cases can affect the mode of delivery.
Clearly, avoiding these complications is critical and generally easy to do by closely monitoring a woman’s TSH level and adjusting thyroid medications to keep the TSH level in a low-normal range (ideally < 2.5 mIU/ml in pregnancy). Women with hypothyroidism typically require increased doses of levothyroxine in pregnancy due to the expanding plasma volume that occurs in pregnancy. Women with hyperthyroidism are usually treated with oral medications, as radioiodine cannot be used during pregnancy.
The big central controversy with thyroid disesae at the present time is whether or not to screen all women who are trying to conceive or newly pregnant. We clearly know that overt over- and underactive thyroid disease is bad. We don’t have solid evidence that subclinical disease poses the same risk, or that treating women with subclinical disease improves these possible risks. There are two main professional societies that regularly review available medical evidence and make recommendations on practice guidelines for physicians. The American College of OBGYN currently does not recommend universal screening, on the basis that there is insufficient data to suggest that treating subclinical hypothyroidism improves obstetric outcomes. The Endocrine Society tends to lean more towards liberal screening, however they also do not recommend universal screening. They instead recommend screening any women with the following characteristics:
2. History of miscarriage or preterm delivery.
3. History of any thyroid dysfunction, or a family history of thyroid dysfunction
4. Presence of a goiter (enlarged thyroid).
5. Known thyroid antibodies.
6. Symptoms suggestive of disease as outlined above.
7. Type I diabetes.
8. Presence of other autoimmune diseases.
9. Prior head or neck radiation.
Most of the patients we see are infertility and recurrent pregnancy loss patients. In our professional opinion, due to the fact that treatment for thyroid disease (especially hypothyroidism) is generally well tolerated and has few risks, we feel that the benefits of treatment outweight the risks of treatment, and therefore screen most of our patients trying to conceive with a TSH level.