Pregnancy and Subclinical Thyroid Conditions

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In our discussion of how the endocrine system changes during pregnancy, we talked about the thyroid. This is the gland in your neck that controls the speed of your metabolism and consequently is of vital import to numerous body processes. We noted that, as the placenta grows and develops, increased quantities of progesterone and estrogen that it releases stimulate the mother’s liver to increase its production of a protein called thyroid-binding globulin (TBG). TBG is one of a few proteins to which thyroid hormones, T3 and T4, can attach as they travel in the bloodstream and in other body fluids. Normally, these thyroid hormones are mostly bound to such proteins, but some amount of the hormones also exist freely in the blood. Based on the concentration of free (unbound) thyroid hormones, the hypothalamus, in the brain, decides whether, and to what degree, the production of thyroid hormones needs to be boosted —just like the thermostat in your house decides when to turn on the air conditioning or heating based on temperature measurements. Extra TBG from the liver means that less thyroid hormone is unbound, which fools the hypothalamus into thinking that there are lower concentrations than there actually are, so the hypothalamus sends out thyrotropin-releasing hormone (TRH) to the pituitary. The pituitary responds to the TRH by boosting its production of yet another hormone, thyroid stimulating hormone (TSH). This is the first hormone that doctors check if they think that you may have a thyroid problem, because it gives a better indication of whether the thyroid is under-or overactive than even the levels of thyroid hormones themselves provide. In pregnancy, however, the boosted TSH is a a product of the changes in the liver, but result of increased TSG, as when you are not pregnant, is that it stimulates the thyroid hormones, which are important, not only for you, but for the normal development of the fetus.

Hyperthyroidism is the state in which your thyroid is overactive, meaning that it is making and releasing too much of the thyroid hormones, T3 and T4. This accelerates your metabolism, producing a range of symptoms and other effects throughout your body, including fertility difficulties, but you also can become hyperthyroid during pregnancy. The most common hyperthyroid condition is Graves disease. Named for the 19th century physician Robert Graves who first described it, Graves disease is characterized by an overactive thyroid gland, due to the immune system producing antibodies  that stimulate the thyroid to release too much of the thyroid hormones. If you have this condition prior to pregnancy, you can worsen, because of pregnancy hormonal changes.

The opposite of hyperthyroidism is hypothyroidism, meaning that thyroid activity that is below normal; the thyroid gland is not releasing adequate amounts of the thyroid hormones, which decelerates your metabolism. If you are hypothyroidism before getting pregnant, but your thyroid is a little bit active, it’s possible that your condition might improve during pregnancy. However, due to the increased need for thyroid activity during pregnancy, it’s also possible that your condition will worsen, or not change much at all. The most common cause of hypothyroidism is an autoimmune condition called Hashimoto thyroiditis, in which the immune system attacks the thyroid until in no longer functions adequately. You also can become hypothyroid if you over-respond to anti-thyroid drugs that are given to treat the opposite condition, hyperthyroidism.

Hyperthyroidism during pregnancy can cause weight loss or a failure to gain enough weight. This puts the fetus is at risk of low-birth-weight, plus there is elevated risk of spontaneous abortion (miscarriage), preterm labor, and stillbirth. Your heart also is at risk if the hyperthyroidism is severe, because they excess thyroid hormones stimulate the heart to beat too fast constantly, which strains your heart. As for hypothyroidism during pregnancy, this decelerates your metabolism, leading to range of symptoms that may include fatigue, feeling sluggish, weight gain (beyond what is expected in pregnancy), muscle weakness, muscle aches and stiffness, joint pain and stiffness, constipation, dry, pale skin, puffy face, hair loss, and brittle nails. If left untreated the condition can deteriorate into an extreme form of hypothyroidism called myxedema, characterized by abnormally low body temperature (hypothermia) and, slowing of multiple organs, and decreasing mental status, ultimately reaching a comatose stage, known as myxedema coma. Hypothyroidism also is associated with an elevated risk of preterm birth and low-birth-weight, low Apgar score, and possibly also the baby being a small size for gestational age. It also can lead to intrauterine fetal death.

While the consequences of hyperthyroidism and hypothyroidism mentioned above are very serious, it is also possible for a pregnant women to have what doctors call a subclinical condition. This means having a certain result on a blood tests that suggests that your thyroid is struggling, either in the direction of hypothyroidism or hyperthyroidism. This all comes down to the concentration of TSH in your blood. As noted at the beginning of this article, TSH is a hormone that your pituitary makes. It’s job is to tell the thyroid to make more thyroid hormones. If TSH in your blood is high, it means that your thyroid is under-active so it is being told to speed up and make more thyroid hormones. On the other hand, if your thyroid is making to much of the thyroid hormones, then TSH is lower than normal; this represents the pituitary telling your thyroid to slow down. Consequently, hypothyroid people have low levels of thyroid hormones and high levels of TSH, while hyperthyroid people have high levels of thyroid hormones and low levels of TSH. However, if somebody has normal levels of thyroid hormones but an elevated level of TSH, this is a clue that the thyroid is struggling to produce those normal hormone levels. It’s a clue that the hormone levels may be dropping too low from time to time, even if the person has not noticed any symptoms, so it’s called subclinical hypothyroidism. Similarly, if thyroid hormone levels are normal but TSH is low, this is called subclinical hyperthyroidism.

While researchers have not found clear evidence that subclinical hyperthyroidism causes problems in pregnancy, subclinical hypothyroidism has been reported to be associated with some adverse conditions, such as preterm birth and even placental abruption (the placenta detaches from the uterus). Consequently, if your TSH is elevated, even if you don’t suffer any symptoms or show any signs of thyroid difficulty, your doctor will take this very seriously, and investigate with various tests.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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