Thyroid Function and Pregnancy

I specialize in thyroid function, but I am also a mother and a grandmother. As a mother, I want to ensure that my daughter has a healthy pregnancy and a healthy baby. For this reason, thyroid function and pregnancy are very much on my mind. Low thyroid function is a primary cause of fertility issues and miscarriages. Many women with undiagnosed hypothyroidism have repeated miscarriages and problems conceiving. While the CDC blames the rise in infertility on women choosing to have children later and toxins in our environment, the women I have spoken to who had several miscarriages, endometriosis, and menstrual irregularities had decades of undiagnosed low thyroid function.

That wasn't the case in the 1940s. In the 1940s, obstetricians and gynecologists agreed that most infertility and menstrual issues could be treated with thyroid therapy. The Mayo Clinic at that time claimed that nearly 70% of women with menstrual irregularities improved with thyroid treatment. At an annual American Medical Association meeting, well-known gynecologist, Robert Frank, said "The sole endocrine preparation that has proved itself a real value has been thyroid extract, which is in use in patients with lowered metabolism." The Johns Hopkins University author of a textbook on gynecology, Dr. Emil Novak, noted that "thyroid therapy for infertility and miscarriage is often more efficacious than any other form of treatment."1 Infertility treatments are a lot more complicated now. There are hormones, surgeries, and drugs that stimulate hormone secretion.

The treatments are incredibly sophisticated, but not a single one addresses the root cause of infertility or miscarriage. In the 1940s, synthetic hormone production was the new frontier. Scientists had synthesized estrogens only a few years before. Medicine inherited the male assumption that a woman's fecundity was limited to the ovaries and uterus. Once the pharmaceutical industry could replicate the steroid hormones produced in those organs, the importance of thyroid function to fertility was quickly forgotten.

Thyroid tests were crude and imprecise in the 1940s. Doctors knew thyroid hormones regulated metabolism, and they used the basal metabolism test--using a machine that measured basal metabolic rate by analyzing the amount of oxygen and carbon dioxide you breathe in and out-- to diagnose someone with thyroid problems. This was the same machine that measured how many calories a person burned, and just as unreliable as the calories in and calories out theory. It didn't account for other metabolic variables like insulin and cortisol. In his textbook, The Thyroid and Its Diseases, published in 1948, Dr. JH Means wrote that the BMR test would be unreliable if the person taking it was under stress.

For this reason, many doctors didn't rely on this test. Instead, Dr. JH Means said the basal metabolism test was "a valuable aid in differential diagnosis,"2 but that doctors should depend on their clinical knowledge of symptoms of hypothyroidism to diagnose their patients. Around 1940, Dr. Evelyn B Man invented the protein-bound iodine test. She used this test to measure thyroid function by measuring the concentration of iodine bound to serum proteins. A world thyroid authority at the time, Dr. Man's research showed that low thyroid function affected cognition and mental health. She demonstrated that hypothyroidism caused many preventable learning disabilities. Her obituary in the New York Times remarked that "many institutionalized patients were found to have been misdiagnosed as demented and were restored to normal functioning with thyroid medication."3 She also published a paper showing the consequence of low thyroid function on the cognitive function of newborns. Congenital iodine deficiency (what used to be called cretinism) has been associated with goiter and iodine deficiency since 1848. But the quantity of iodine added to salt was barely enough to prevent goiter, the swelling of the thyroid gland, not hypothyroidism. Unfortunately, this test was also somewhat imprecise, as iodine could be bound to other proteins not associated with thyroid hormones. A full work-up for thyroid function then included the basal metabolic rate test, an iodine uptake test, and the protein-bound iodine test. In a person with hypothyroidism, the BMR test demonstrated a reduced level of tissue and cellular activity; iodine uptake was slower, and protein-bound iodine levels were low.

Thyroxine (T4) was isolated in 1917, and Triiodothyronine (T3) was isolated in 1959. The ability to measure these two thyroid hormones (there are more—T1, T2, and calcitonin) came in the 1970s. Still, as 99% of T4 was bound to proteins, biochemists realized it wasn't as good a measure of thyroid function as the level of free, unbound hormones. These would truly indicate the level of usable thyroid hormones. Free T4 and T3 radioimmunoassays weren't routinely available until the early 1980s. However, by this time, the TSH test, invented by Robert Utiger in 1973, became the single test used to measure thyroid function.

Thyroid testing is a problematic subject. Possibly because of the long, slow history of its development (and this evolution isn't even finished until T1 and T2 tests are available), testing is fraught with misinformation, confusion, and just plain marketing. The result ranges are too wide. The free hormones are often not tested. There is too much emphasis on testing TSH and T4. TSH is only sensitive to T4 levels in some, not all, tissues. T3 level needs vary widely in different tissues, situations, and people, as Robert Utiger found out in research papers from the 1970s into why TSH was not sensitive to T3 levels. Pregnancy is a good example of that. This is why the importance of proper testing in pregnancy cannot be ignored.

Still, the importance of proper testing in pregnancy cannot be ignored. Thyroid textbooks have all historically had chapters discussing the risks of low thyroid function for women wanting to conceive. J.H. Means' textbook, Thyroid and Its Diseases, cited above, has a chapter titled "Sterility and Habitual Abortion." Most general practitioners, gynecologists, and even some endocrinologists haven't researched the dramatic changes in thyroid secretion and conversion during pregnancy. The same goes for treatment. For this reason, with much annoyance to my daughter, I am annoyingly preoccupied with her thyroid function during pregnancy. I am not concerned about her nutrition. She has been mostly carnivore for years and was aware that during the first trimester, spikes of progesterone brought along with them carbohydrate cravings and nausea.

But I'm always thinking about her thyroid. The thyroid is the first endocrine organ formed in the fetus, simultaneously with the formation of the central nervous system and the neural tube, depending on a constant supply of iodine. She, like the rest of my family, has been taking Lugol's iodine solution for years, so I'm not worried that the measly 150 mcg of "iodine" in the prenatal vitamins her general practitioner gave her will be a drop in the bucket to ensure her iodine needs during pregnancy. Most research papers indicate that the iodine needs of a pregnant woman are above 250 mcg, and potassium iodide, the kind of "iodine" in prenatal vitamins, isn't even iodine.

Pregnancy has enormous effects on the thyroid and thyroid function. Thyroid hormone requirement increases from 20% to 40%. Placental hormones suppress TSH. Free T4 and Free T3 hormone secretion and circulation increase. Unfortunately, most doctors use non-pregnant reference ranges for pregnant women, which means many women will not be diagnosed if they are hypothyroid. Hyperthyroidism is rare in pregnancy. The ranges for euthyroid (healthy thyroid) levels change, and many other ranges change, such as creatinine, ferritin, iron, and homocysteine. Furthermore, iodine levels increase five times in women who are iodine-sufficient.

Unaddressed hypothyroidism poses dangerous risks for fetal health. Even mild hypothyroidism affects fetal brain development. Maternal thyroid hormones cross the placenta to form the fetal brain. The babies of women unable to increase their production of thyroid hormone in early pregnancy are at risk for neurological disabilities. Fetal hypothyroidism can cause autism, learning and motor disabilities, and lower cognitive outcomes. Not only is the fetal brain affected, but there are increased risks of stillbirth, low birth weight, and fetal mortality. For the mother, hypothyroidism causes miscarriage, anemia, hemorrhaging, preeclampsia, fatigue, stress, and postpartum depression.

Why am I still concerned, even if my daughter has been taking Lugol's iodine solution for a decade? Well, I'm a mom, and I want her doctor to test her thyroid function every time they test everything else at her antenatal visits. They are only testing TSH and free T4 because she asks for it. Thyroid screening for all pregnant women to optimize cognitive development in newborns has been debated for decades. You can almost hear the researchers begging for it in numerous papers, but the idea is always swatted away with percentages that favor insurance companies. If there is a history of thyroid disorder in the family, you can get doctors to agree to test TSH and sometimes both TSH and free T4, which doesn't help if they use non-pregnancy ranges as a comparison. It is, however, also crucial to test free T3, which they flat-out refuse to do, so my daughter does it privately. Antenatal care should also screen for thyroid antibodies because autoimmune thyroid issues often appear for the first time in women during pregnancy.

I want to be sure, for my grandchild's sake, that my daughter has plenty of thyroid hormones to develop her child's brain. It will be able to make enough thyroid hormones after 18-20 weeks. But I also want to be sure, for both their sakes, that there are no complications. She needs plenty of energy to live her life during the pregnancy--to keep her stress levels low with that energy. With optimal thyroid levels, the birth will go smoothly without the danger of hemorrhaging. Her skin will have the elasticity to stretch during labor so that she won't have any perineal tears. It willalso return to normal after birth so that she won't end up with the stretch marks many women think are normal with pregnancy, for which they undergo "mommy makeovers." I am excited for her to feel the joy that comes with having a baby without worrying about the danger of postpartum depression, which is so common in hypothyroid women.

My daughter is planning a natural birth. There will be some discomfort. There's no way to be pregnant without a degree of stress. Yet, if her thyroid levels are optimal, she is protected from many risks.