The most common cause of hypothyroidism is the presence of elevated levels of antibodies directed to the thyroid gland (this used to be known as Hashimoto’s thyroiditis). It is possible, however, to have elevated levels of anti-thyroid antibodies and still have a normally functioning thyroid gland. In this case, it is important to monitor the thyroid hormones on a regular basis since these women are at greater risk for hypothyroidism in the future. They must also be checked in pregnancy and after delivery since these are times when a woman may be more prone to thyroid malfunction.
Anti-thyroid antibodies and miscarriage
Women with anti-thyroid antibodies and a normally functioning thyroid gland may be at greater risk for miscarriage. Some studies looked at women with recurrent miscarriage and found that they have higher levels of anti-thyroid antibodies compared to women without recurrent miscarriage. Some studies did not find an association. One publication combined the results of 18 studies on this topic (meta-analysis) and concluded that there is an ASSOCIATION between the presence of elevated anti-thyroid antibodies and miscarriage. One of the most recent studies on the topic, which has also been one of the largest, again showed a correlation between thyroid antibodies and miscarriage. There are several explanations for why there may be an association between anti-thyroid antibodies and miscarriage.
- Link with other autoimmune problems.
It is possible that women with elevated levels of anti-thyroid antibodies may have other immune problems and these other problems may be responsible for miscarriage. It is well accepted, for example, that another type of immune problem involving anti-phospholipid antibodies can cause miscarriage. Some of the studies did try to control for this problem by excluding women who had abnormal levels of other known antibodies but there still may be immune issues that they did not control for.
- Direct involvement of the antibodies.
A commonly asked question is why antibodies against the thyroid gland would cause a problem elsewhere. It may be possible that the anti-thyroid antibodies may “get confused” or “cross react” and therefore also attack placental or fetal tissues. If this hypothesis were true, we should see a “dose-response relationship” That is, we would expect higher levels of anti-thyroid antibodies in women who actually miscarried compared to those who had a full term delivery. Most studies did not look at this. Two studies did not find a dose response relationship and one did.
- Effect of age
In most of the studies analyzed, women with elevated levels tended to be older as a group compared to those women with normal antibody levels. Since the miscarriage risk for women increases with age, this could be an explanation.
- Sub-clinical hypothyroidism
Women with anti-thyroid antibodies are at higher risk for developing clinical hypothyroidism. They also have a higher rate of sub-clinical hypothyroidism. Some studies show that even when thyroid function is in the normal range that women with antibodies may have a slightly higher TSH level than women without antibodies.
Treatment of anti-thyroid antibody related miscarriage
Very well designed trials indicate that selenium supplementation may reduce the levels of some types of anti-thyroid antibodies. It is possible that if the levels of the antibodies can be reduced, then the risk of miscarriage may also be reduced The effects of selenium in pregnancy were recently evaluated in a study performed in Italy. Approximately 150 pregnant with with elevated levels of one group of thyroid antibodies were split into two equal groups. One group received 200 micrograms of selenium and the other group received placebo. During pregnancy and post-partum, more women who took the placebo were more likely to show evidence of thyroid malfunction and need thyroid hormone supplementation.
Additional information about selenium
Thyroid hormone supplementation
One very small study found that low dose supplementation of thyroid hormone reduced the miscarriage risk when compared to another common immune therapy. Another larger study did not find a benefit. In this study, women who were positive for anti-thyroid antibodies were randomly assigned to receive either thyroid hormone or placebo. These two groups were compared to a third group who were negative for anti-thyroid antibodies. All the patients in the study underwent IVF to achieve pregnancy.
The study findings were as follows :
- The pregnancy rate with IVF was similar in all groups. In other words, the presence of anti-thyroid antibodies did not influence the chances for success and using thyroid hormone did not improve the chances for success.
- The miscarriage rate with IVF was higher in women who had anti-thyroid antibodies. Using thyroid hormone for treatment did not reduce the risk for miscarriage.
The largest study to date involved over 1000 women. Women with elevated thyroid antibody levels were compared to women with normal levels. Those women with thyroid antibody elevations were divided into two groups. One group received thyroid hormone, the other did not. The results of the three groups were then compared. The women who did not have thyroid antibody elevations and the women who received thyroid hormone had lower miscarriage rates. The group with thyroid antibodies that did not receive thyroid hormones had a much higher risk for miscarriage. Statistical analysis showed that these results were unlikely to be due to chance.
At this point, therefore it does seem reasonable to use selenium and thyroid hormone as a means to reduce miscarriage risk in women with thyroid antibodies.