Understanding abnormal thyroid function during pregnancy

Refresh your understanding of the relationship between TSH and abnormal thyroid function tests during pregnancy.

Tracy Tylee, MD
Tracy Tylee, MD
27th Feb 2018 • 2m read

Pregnancy can wreak havoc on thyroid function. In this video from our Thyroid Disease Masterclass, Dr Tracy Tylee explains the relationship between TSH and gestational thyrotoxicosis, and factors that lead to abnormal thyroid function tests in pregnant women.

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Master the management of thyroid disease in our Thyroid Disease Masterclass. Dr. Tracy Tylee–Clinical Assistant Professor of Endocrinology at the University of Washington–will review the pathophysiology of thyroid dysfunction, discuss the interpretation of thyroid tests, explain how pregnancy and certain medications can alter thyroid function, and evaluate the best treatment options.

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Video Transcript

[00:00:00] TSH and hCG are structurally very similar. They have the same alpha subunits but slightly different beta subunits. Because of the similarity, the high levels of hCG, seen with pregnancy, connect at the TSH receptors on the thyroid and increase thyroid hormone production. This increases inhibition at the level of the pituitary and results in lower TSH levels. Thus, the labs in early pregnancy can

[00:00:30] show high thyroid hormone levels associated with a low TSH, consistent with hyperthyroidism. This condition is known as gestational thyrotoxicosis. It is most commonly seen in conditions with high hCG levels such as multiple gestations or hyperemesis gravidarum or severe morning sickness. You can see here that the hCG levels peak around week ten of pregnancy and then decrease for the duration of pregnancy. As the hCG levels come down, the TSH levels

[00:01:00] return to normal and remain normal for the rest of pregnancy. Thus, gestational thyrotoxicosis is a very limited process and will resolve, thus, it does not require any treatment. Iodine metabolism is also changed during pregnancy. Normally, iodine is taken up into the cell, where it's formed into thyroid hormone, which is then secreted into the bloodstream. During pregnancy, there's an increase in renal iodine loss, due to increase filtration at the level of the kidneys. In addition, there's an increase in iodine needs,

[00:01:30] due to fetal thyroid hormone production. Pregnant women and women who are breastfeeding have significantly higher iodine needs than non-pregnant women. If dietary iodine intake is insufficient, there'll be low thyroid hormone production. This results in an increase in TSH levels. Labs, in this case, will show low thyroid hormone with an increase in TSH, consistent with the diagnosis of hypothyroidism. Iodine is important, both for maternal thyroid hormone production and fetal

[00:02:00] thyroid hormone production. So, if there is inadequate iodine intake during pregnancy, thyroid hormone levels will be low for both mother and baby and can result in severe neurologic and growth complications. Because of the importance of iodine in pregnancy, the World Health Organization has undertaken a program of salt iodization in areas at risk of iodine deficiency. In addition, women can take a prenatal vitamin containing iodine, to help ensure adequate iodine intake during pregnancy.