What are the normal changes between thyroid function and pregnancy?
Changes in thyroid hormones. Changes in thyroid function are very important for a normal pregnancy. Under normal circumstances, TSH is usually slightly low in the early stages of pregnancy due to high chorionic gonadotropin levels (slow release of the hormone during the early tests of pregnancy) and returns to normal later. The increase in total T4 is usually due to an increase in serum protein due to estrogen, but free thyroxine measurements remain normal. If TSH is normal in the function of the thyroid, then free T4 and free T3 are normal throughout pregnancy.
Changes in the size of the thyroid gland, which may increase in size during pregnancy (enlargement of the thyroid = goiter), especially in iodine-deficient parts of the world. The thyroid is often only 10-25% enlarged. However, sometimes over-checking of thyroid function can promote the formation of very visible nodules in the thyroid.
What is the interaction between the thyroid function of the mother and the baby?
During the 10th-12th weeks of pregnancy, the fetus first becomes completely dependent on the mother’s thyroid hormone production. By the third trimester, the fetal thyroid gland begins to secrete thyroid hormones. However, the fetus is still dependent on the mother for adequate amounts of iodine, which is necessary for the synthesis of thyroid hormones. In most areas of China, the normal diet contains sufficient iodine, so no additional iodine supplementation is needed. However, for iodine deficient areas, proper food and salt supplementation is necessary to prevent the development of iodine deficiency disorders.
Hyperthyroidism
What are the most common causes of hyperthyroidism during pregnancy?
Overall, the most common cause of maternal hyperthyroidism in 80-85% of women is Graves’ disease (see Graves’ disease manual), and there are 1500 cases of hyperthyroidism in women each year. Diagnosis of hyperthyroidism during pregnancy is very difficult and thyroid 131I scans are inaccurate due to low iodine uptake during pregnancy, which is mostly concentrated in the fetus. Therefore, the diagnosis of hyperthyroidism during pregnancy is based on a detailed medical history, physical examination and laboratory tests.
What are the risks for mothers with Graves’ disease/hyperthyroidism?
In addition to the typical symptoms associated with hyperthyroidism, improper treatment of patients with maternal hyperthyroidism can lead to serious complications such as preterm delivery or pre-eclampsia.
What are the risks of Graves’ disease/hyperthyroidism to the fetus?
1. Uncontrolled hyperthyroidism: Uncontrolled hyperthyroidism may be associated with fetal tachycardia (rapid heart rate), small for gestational age fetuses, preterm birth, stillbirth and congenital malformations. This is another important factor in the treatment of maternal hyperthyroidism.
Excessive elevation of thyroid-stimulated globulin (TSI) levels: Graves’ disease is caused by the production of antibodies to thyroid-stimulated TSI. These antibodies can pass through the placenta and have an effect on the fetal thyroid gland. Increased maternal TSI levels have been shown to cause fetal or neonatal hyperthyroidism, but this is very rare (2-5% of cases of Graves’ disease during pregnancy). In mothers with Graves’ disease, TSI is usually detected in the third trimester.
It is very rare for maternal TSIs to cause fetal hyperthyroidism in women treated with antithyroid drugs, since antithyroid drugs can cross the placenta into the fetus. Premature treatment of mothers with Graves’ disease is potentially risky for the fetus (e.g., radioactive iodine or surgery) because the mother does not need antithyroid medication. If you have been treated for thyroid disease in the past, it is very important to tell your doctor in charge so that better measures can be taken to ensure the health of the fetus during pregnancy.
3. Anti-thyroid medication (ATD). Methimazole (Tabazol) or propylthiouracil (PTU) are used to treat hyperthyroidism. All of these drugs directly affect the thyroid function of the infant through the placenta. Empirically, propylthiouracil has been the drug of choice for the treatment of maternal hyperthyroidism, but recent studies have shown that both drugs are safe for use during pregnancy. The use of ATD in the treatment of maternal hyperthyroidism should minimize the effects of the drugs on the fetus. Neither drug has an increased risk of causing fetal defects to the fetus.
What are the treatment options for maternal Graves’ disease/hyperthyroidism?
Mild hyperthyroidism usually does not require close monitoring as long as both maternal and fetal treatment are well managed. Sometimes anti-thyroid medication is necessary as the treatment of choice (see above). The goal of this therapy is to keep the antithyroid medication in the mother at the lowest possible dose: free T4 and free T3 in the normal range or slightly higher. This therapy should be followed by close monthly monitoring of thyroid function during pregnancy.
In women who cannot receive antithyroid medication, surgery is an option and an alternative, (i.e., surgery should be performed in hypersensitive women).
Radioactive iodine therapy is contraindicated in hyperthyroidism during pregnancy because it easily crosses the placenta and is taken up by the baby’s thyroid gland. This can lead to permanent glandular destruction due to hypothyroidism.
Beta-blockers can be used to treat palpitations and tremors caused by hyperthyroidism during pregnancy. In general, these medications should only be used until the symptoms of hyperthyroidism are controlled by anti-thyroid medications.
What are the complications of Graves’ disease after delivery?
Graves’ disease usually worsens 3 months after delivery, during which time the need for antithyroid medication doses often increases. As always, regular monitoring of thyroid function tests is necessary during this time.
Can a woman taking antithyroid medication for her disease breastfeed her baby?
Yes. Propylthiouracil (PTU) is the drug of choice because it is high in protein. Therefore, only a lower dose of propylthiouracil (PTU) passes into breast milk compared to tapazole. It is important to note that infants will periodically need their own or their mother’s thyroid function to ensure that their normal thyroid metabolism is maintained.