The thyroid gland during pregnancy
Before the main character takes the stage, let’s briefly introduce the background of the episode: what changes does the thyroid gland undergo when it enters pregnancy?
The thyroid gland’s main function is to concentrate iodine throughout the body and process it into the essential thyroxine. During pregnancy, the body’s basal metabolic rate increases significantly and the thyroid gland has to work overtime to expand its production capacity to meet the physiological needs of the mother and fetus. At this time, the size of the thyroid gland increases by about 10% compared to the pre-pregnancy period, and if the supply of iodine as a raw material is insufficient at this moment, as in the case of pregnant women living in iodine-deficient areas, the size of the thyroid gland increases even more, by 20%-40%.
In addition to morphological changes, the level of hormones in the body of a pregnant woman changes significantly. From the beginning of pregnancy, the placenta synthesizes a large amount of human chorionic gonadotropin (HCG), and it is this principle that is used in the early pregnancy test to determine whether a woman is pregnant by testing the level of HCG in her urine. An important function of HCG is to mimic the action of thyroid stimulating hormone (TSH) and stimulate more thyroxine secretion. At the same time, the level of thyroid binding globulin (TBG) increases significantly in response to the elevated estrogen in pregnant women, which also causes an increase in thyroxine levels.
There are so many unfamiliar and awkward terms that some people may already feel overwhelmed, so let’s just skip to the summary.
After pregnancy, especially in the early stages of pregnancy, the changes that occur in the organism are.
1. increase in basal metabolic rate (manifested by increased skin temperature, fear of heat and sweating, excessive food and hunger, increased heart rate, etc.)
2. elevated levels of thyroxine (mainly elevated TT3 and TT4, mildly elevated FT3 and FT4 in the early stages and decreased in the middle and late stages) and decreased levels of thyroid stimulating hormone (TSH).
3. The thyroid gland may be mildly enlarged (often not discernible to the naked eye).
Transient hyperthyroid syndrome during pregnancy
As many of you can see, these changes are very similar to hyperthyroidism. There are even a few pregnant women (about 1 to 3 percent) who are so obvious that they come to the clinic with “hyperthyroidism”.
Therefore, the first thing that doctors need to do in their clinical work is to distinguish correctly whether this is a physiological phenomenon caused by pregnancy or a pathological “hyperthyroidism” caused by a disease (e.g. Graves’ disease).
The former is called “Gestational Transient Hyperthyroidism Syndrome (GTH)”, which is a physiological change that occurs mostly in early pregnancy.
If the following characteristics are met, it is likely to be GTH, so do not rush to use medication.
1. Diagnosed with “hyperthyroidism”: FT4 and TT4 are elevated and TSH is <0.1mIU/L.
2. No history of hyperthyroidism before pregnancy.
3. Negative thyroid autoantibodies.
4, no obvious goiter and no combined thyroid eye disease.
5, occurs in early pregnancy, sometimes combined with severe vomiting, “hyperthyroidism” symptoms are mild, and can gradually resolve with the progress of pregnancy.
Graves’ disease in pregnancy
If you don’t need medication, don’t use it indiscriminately. Conversely, when medication is needed, do not stubbornly resist it, even at this particular time of pregnancy.
The most common disease causing hyperthyroidism in pregnancy is diffuse toxic goiter (Graves’ disease, GD), which has a prevalence of 0.1-1% in pregnant women, and with such a high population base in China, the number of patients is definitely not small.
The possibility of GD must be highly suspected if there are the following characteristics.
1, combined with ocular symptoms, such as infiltrative proptosis
2. presence of diffuse goiter with localized vascular murmur and tremor
3. Positive thyroid autoantibodies, especially TRAb, which has a sensitivity of 95% and specificity of 99%.
The importance of using antithyroid drugs is emphasized for two reasons based on the following.
1. hyperthyroidism, if left untreated, can cause serious adverse effects on the pregnant woman and fetus, such as miscarriage, premature birth, hypertension, heart failure, fetal growth restriction and abnormal development, etc.
2. The drugs are relatively safe, although there are some risks, but the benefits outweigh the disadvantages on balance.
Currently, there are two main types of antithyroid drugs: methimazole (MMI) and propylthiouracil (PTU). Compared to MMI, PTU has a shorter half-life, lower placental passage rate and less impact on the fetus, so doctors have long used PTU as the drug of choice for treating hyperthyroidism in pregnancy. However, more and more studies have recently found that PTU has the potential to cause side effects such as liver cell damage and vasculitis during treatment, while MMI appears to be much safer in this regard.
All things considered, the current medication consensus is as follows, which mothers-to-be with hyperthyroidism are asked to keep in mind.
1. before pregnancy, choose methimazole (MMI) and switch to propylthiouracil (PTU) as soon as they enter pregnancy preparation.
2. early in pregnancy, within 12 weeks of pregnancy, choose propylthiouracil (PTU).
3. in the middle and late stages of pregnancy, stop PTU and switch to methimazole, with a switching ratio of 100 mg PTU to approximately 10 mg MMI.
4. throughout pregnancy, to avoid drug effects on the fetus, the lowest drug dose should be used, not in combination with thyroxine, with the control goal of making the pregnant woman’s FT4 close to or mildly above the upper limit of normal values.
5. During breastfeeding, choose methimazole (MMI), which should be taken in divided doses, and choose to take the drug just after the baby has finished breastfeeding. Special emphasis: the drug is basically safe for the baby’s development, and breast milk is precious, so please feel free to breastfeed your hyperthyroid mother.
Although mothers-to-be already have a hard time, if they develop hyperthyroidism, they must also work even harder to have regular review for the health and safety of themselves and their babies.
It is recommended that liver function be checked every 4 weeks and FT4 and TSH be rechecked every 2-6 weeks depending on the condition.
There is also a recommended test: TRAb, which means that if it changes from positive to negative, it means that medication can be discontinued; if it stays high, the fetus must be closely monitored from the middle of pregnancy, for example, by ultrasound, monitoring fetal heart rate, amniotic fluid volume and fetal goiter, and newborn screening for hyperthyroidism should be performed after birth.
My old classmate NumB once fondly and profoundly admonished me, “The conclusion must be written once again very clearly at the end of the scientific article, because many people are like me and skip the boring body and go straight to the end.”
In order to thoroughly implement the spirit of NumB’s important instructions and to comprehensively promote the reform of the science writing style, the following is a summary of this article.
Do not worry about hyperthyroidism in pregnancy, it is most important to identify the cause.
GTH is not dealt with, but Graves requires medication.
The first choice is methimazole to avoid liver damage, and propyl sulfide to protect the baby at the beginning of pregnancy.
The frequency of monthly review is not suspected, and safety depends on small doses.