How to diagnose hypothyroidism during pregnancy? Should I take iodine supplements?

Author: Xiao Jianzhong, Chief Physician of Beijing Tsinghua Chang Gung Hospital affiliated to Tsinghua University

Reviewer: Wu Xueyan, Chief Physician, Peking Union Medical College Hospital

Thyroid hormone is a very important hormone that maintains the body's metabolism. Patients with hypothyroidism who lack thyroid hormone often feel cold and fatigued. Some patients may also experience edema, weight gain, and dry skin. Of course, many patients with hypothyroidism may not have any symptoms and are completely discovered through laboratory tests.

Under normal circumstances, after a woman becomes pregnant, the body's demand for thyroid hormone will increase significantly. Therefore, some pregnant women, especially those with Hashimoto's disease, have normal thyroid function before pregnancy, but will experience hypothyroidism after pregnancy.

Diagnosis of hypothyroidism during pregnancy mainly relies on thyroid function tests. We usually require the measurement of thyroid stimulating hormone (TSH), free thyroxine, i.e. FT4, FT3, and total T3 and T4 levels. If TSH is high and FT4 is low, it is called hypothyroidism. Sometimes only TSH is elevated and FT4 is not low, which is called subclinical hypothyroidism (a milder form of hypothyroidism).

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The second thing is to see the cause of hypothyroidism. You may need to do a thyroid antibody test, such as thyroid peroxidase antibodies (TPO antibodies) and thyroglobulin antibodies (TG antibodies).

As for how to diagnose whether a pregnant woman has hypothyroidism, this involves determining the normal value of thyroid hormone.

In the past, we set the normal value of TSH at below 2.5mIU/L, believing that as long as TSH exceeded 2.5mIU/L, the obstetrician would ask the pregnant woman to see an endocrinologist to determine whether she had hypothyroidism.

Now it is found that many pregnant women with this value are not hypothyroidism at all, and it is found that if these pregnant women are treated, it does not bring much benefit. Therefore, the current cut-off value has been raised both internationally and domestically to 4.0mIU/L.

At this time, we need to look at this matter comprehensively. For example, this pregnant woman's TSH value may be between 2.5-4.0mIU/L, but TPO antibodies and TG antibodies are elevated, which means that the thyroid gland is inflamed. Because during pregnancy, the demand for thyroid hormone increases, drug treatment is not required at this time, but changes in the hormone levels of pregnant women should be monitored to see if it will develop.

There is another situation in which TSH is normal during pregnancy but FT4 is reduced. This condition is called hypothyroidism.

In the past, we did not have a particularly deep understanding of this situation. We thought that low FT4 was also hypothyroidism, so we took thyroid hormone supplements. However, we later found that thyroid hormone supplements were not very useful for these pregnant mothers, mainly because the body's metabolism had changed.

Now, if it is a simple low T4, we do not treat it. Why not treat it? Because a large number of studies have found that simple low T4 has no effect on the mother and the fetus, and it is difficult to make up for it, so we generally do not advocate treating isolated low T4 syndrome, or low thyroid syndrome.

Only when FT4 is reduced and TSH is increased, treatment is needed. Because many studies now tell us very clearly that pregnant mothers with hypothyroidism may experience miscarriage, abnormal fetal development, and even some impact on the child's intellectual development. So at this time, we should take a more active attitude to intervene and treat.

For women who are clearly hypothyroid during pregnancy, taking medicine is very necessary. The drug for treating hypothyroidism is called levothyroxine sodium tablets, which has the same structure as T4 (thyroxine) secreted by our body, so it only supplements this substance that the human body lacks. If the amount of supplementation is appropriate, it will not cause any impact on the mother and fetus.

If you take too much levothyroxine sodium tablets, it will also cause some problems, such as drug-induced hyperthyroidism. So this requires us to supplement appropriately and reasonably, and to check regularly, and adjust the dosage of levothyroxine sodium tablets by testing indicators.

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Some friends are also very concerned about whether iodine supplementation is needed for hypothyroidism during pregnancy.

Hypothyroidism during pregnancy indicates that the pregnant mother's ability to synthesize thyroid hormone is reduced.

In the past, one of the important causes of hypothyroidism was insufficient iodine nutrition. There was a disease called endemic goiter, which was caused by insufficient iodine nutrition in the area, thus causing hypothyroidism.

However, this situation is very rare now, because there is basically iodine in table salt, there is also some iodine in drinking water, and seafood also contains iodine, so there are very few people who are iodine deficient.

Therefore, hypothyroidism nowadays is generally not caused by iodine deficiency. Most of the time, it is caused by thyroid inflammation, surgery, or other factors. Therefore, iodine supplementation is not very useful at this time.

But we also have to pay attention to one thing. At about 8 weeks of pregnancy, the fetus's thyroid gland begins to develop. The fetus needs iodine, so adequate iodine nutrition is very important for the fetus. At this time, eating more seafood can help supplement the fetus' iodine nutrition.