Example 1: I am 30 years old and have had hyperthyroidism for nearly four years. I have been taking thiamazole regularly, and my condition is now fully controlled. Following my doctor's advice, I stopped taking the medication six months before becoming pregnant. Initially, everything was fine, but later my hyperthyroidism recurred. The main symptom is a fast heart rate, around 100 beats per minute, which increases to 120-130 beats per minute with even slight exertion.Should I take medication? Will it affect the fetus?
Example 2: I am over four months pregnant and suddenly experience excessive sweating, increased appetite, hand tremors, and apparent weight loss. Hospital tests revealed hyperthyroidism. Should I continue the pregnancy or terminate it?
Example 3: My husband is an only child, so we face significant pressure to have a child. However, I have hyperthyroidism that has not been cured. I currently take propylthiouracil, one tablet daily. What should I do? Is it safe to risk pregnancy?
As an endocrinologist, I encounter cases similar to the above almost daily.Hyperthyroidism is a common condition, with women of childbearing age being the most susceptible group. Consequently, its impact on fertility has garnered increasing attention.
The effects of hyperthyroidism on pregnancy can be divided into two aspects: the disease's direct impact on the mother and fetus, and the effects of the treatment received on both.
Theoretically, mild hyperthyroidism has no significant impact on pregnancy. However, moderate to severe hyperthyroidism, or uncontrolled symptoms, are associated with increased rates of miscarriage, gestational hypertension (preeclampsia), preterm birth, small-for-gestational-age infants, and perinatal mortality.The exact mechanisms underlying hyperthyroidism's impact on pregnancy remain unclear. It may be attributed to excessive consumption of nutrients due to hyperthyroidism and the higher incidence of preeclampsia, which can impair placental function. Due to the placental barrier, only small amounts of T3 and T4 can cross the placenta, thus not causing neonatal hyperthyroidism.
Pregnancy itself has minimal effect on the severity of hyperthyroidism; conversely, pregnancy often leads to varying degrees of improvement in the condition.However, severe hyperthyroidism during pregnancy can exacerbate pre-existing cardiac pathology due to the increased cardiac workload. In rare cases, childbirth, postpartum hemorrhage, or infection may trigger a thyrotoxic crisis. Overall, advances in medical care have significantly reduced the adverse effects of hyperthyroidism on pregnancy.
Treatment for hyperthyroidism falls into three categories: oral medication, radioactive iodine therapy, and surgery. Radioactive iodine therapy carries risks of fetal malformations and permanent hypothyroidism, making it generally contraindicated during pregnancy. While surgical risks have decreased with improved techniques at major hospitals, uncertainties surrounding anesthesia and patient conditions make surgery generally discouraged during pregnancy.
Drug therapy remains the most common and safest approach, particularly with propylthiouracil (PTU). Current research indicates that while PTU crosses the placenta, its teratogenic effects are minimal, especially at low doses (below 100 mg daily, equivalent to 2 tablets per day in standard formulations), making it safer.
Since thyroid hormones do not readily cross the placenta, the primary goal of drug therapy is to control maternal hyperthyroidism, preventing complications like thyrotoxic crisis and preterm delivery. The clinical target is typically maintaining maternal free T4 (FT4) levels below 1.4 times the upper normal limit, corresponding to mild hyperthyroidism. Antithyroid drugs are only used when levels exceed this threshold.
Applying the above principles, let us analyze the three scenarios mentioned earlier:Examples 1 and 2 should first undergo hospital evaluation to determine the severity of hyperthyroidism before deciding on further treatment. Example 3 actually represents the optimal time for clinically hyperthyroid women of childbearing age to conceive: taking low-dose antithyroid medication poses no harm to either mother or fetus, while preventing recurrence of hyperthyroidism and ensuring a smooth pregnancy.Therefore, endocrinologists generally advise women with hyperthyroidism to conceive while taking low-dose antithyroid medication (propylthiouracil) and maintaining well-controlled hyperthyroidism.
Patients should be particularly aware of the following: ① Due to physiological changes during pregnancy, total T3 and total T4 (typically labeled TT3 and TT4 on lab reports) cannot accurately assess the condition. Therefore, diagnosing hyperthyroidism in pregnant women requires testing free T3 and free T4 (typically labeled FT3 and FT4 on lab reports).② Beta-blockers like propranolol (Inderal) may increase uterine muscle tone, leading to placental insufficiency and fetal growth restriction. Therefore, they should not be used for treating hyperthyroidism during pregnancy.③ When managing hyperthyroidism in pregnant women, thyroid function must not be suppressed excessively. The goal is to achieve normal or mild hyperthyroidism. Hypothyroidism poses greater risks to both the mother and fetus than hyperthyroidism itself. Therefore, pregnant women with hyperthyroidism require frequent thyroid function monitoring. ④ Radioactive iodine therapy frequently induces hypothyroidism. Consequently, women who are not yet pregnant but wish to conceive should generally avoid undergoing this treatment.⑤ Hyperthyroidism may slightly affect sperm production in men but generally does not cause sperm abnormalities. Thus, its impact on the fetus is minimal. ⑥ Antithyroid medications can affect an infant's thyroid function through breast milk. Therefore, breastfeeding is not recommended for mothers with hyperthyroidism.⑦ Given the complex effects of hyperthyroidism on pregnancy, it is advisable to consult an endocrinology specialist at a reputable hospital for professional guidance.
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