Clinical Thyroidology® for the Public

Summaries for the Public from recent articles in Clinical Thyroidology
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THYROID AND PREGNANCY
Mild abnormalities in thyroid hormone levels in one blood test in early pregnancy may become normal when repeated.

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BACKGROUND
Thyroid hormone from the mother is essential for a baby’s normal development during pregnancy. Importantly, thyroid hormone levels in pregnant women change during pregnancy. Thyroid hormone requirements increase during pregnancy and this may result in an increased TSH and possibly hypothyroidism. Pregnancy hormones (HCG) can stimulate the thyroid, causing thyroid hormone levels to increase and TSH to decrease. Most of the time, these changes are short-lived and resolve without therapy but some do need to be treated. However, some people may not be able to appropriately respond, leading to diagnosis of either hypothyroidism or hyperthyroidism. Abnormal thyroid hormone levels, both high and low, in mothers during pregnancy have been associated with harmful outcomes for pregnancy and baby’s development. Therefore, it is important to correctly diagnose thyroid disease in pregnancy. Hypothyroidism or hyperthyroidism during pregnancy is often diagnosed based on a single blood test. This study investigated how often the diagnosis of thyroid problems remains the same when thyroid hormone levels in pregnant women were checked more than once during early pregnancy.

THE FULL ARTICLE TITLE
 Knøsgaard L et al 2022 Classification of maternal thyroid function in early pregnancy using repeated blood samples. Eur Thyroid J 11(2):e210055. PMID: 34981754.

SUMMARY OF THE STUDY
A total of 1466 pregnant women from a national Danish database were included in the study. The authors collected the results of thyroid blood tests from the database, including thyroid stimulating hormone (TSH), free thyroxine (FT4), thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TgAb) levels. Women had initial blood tests at an average of 8 weeks of pregnancy and repeat blood tests at an average of 12 weeks of pregnancy. Hypothyroidism or hyperthyroidism were defined using the pregnancy-specific normal ranges established from this population.

About 39% of women who were classified as having thyroid problems in the first measurement again had thyroid problems in the second measurement, so in 61% the problems resolve. Only 18% of pregnant women who had low FT4 levels only in the first measurement had again low FT4 in the second measurement. On the other hand, 88% of women who had high levels of thyroid antibodies (TPOAb or TgAb) in the first measurement had high levels of thyroid antibodies in the second measurement.
Those with lower TSH level (<0.01mIU/L in hyperthyroidism) or the higher TSH level (>7.0mIU/L in hypothyroidism) in the first measurement were more likely to have persistent thyroid abnormalities in the second measurement, compared to those who had only mild thyroid abnormalities. Thyroid problems were more likely to continue from the first to the second measurements if women had high thyroid antibody levels.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
In this study, the diagnosis of hypothyroidism or hyperthyroidism did not persist in more than half of pregnant women when blood thyroid levels were repeatedly measured several weeks apart in early pregnancy. Only about 2 in 5 women who had thyroid problems identified in the first blood tests still had thyroid problems in the second blood tests. Very high or very low levels of TSH and presence of thyroid antibody increased likelihood of having persistent thyroid dysfunction.

Given many studies showing adverse effects of thyroid problems in pregnant women on pregnancy and baby’s developmental outcomes, it would be important to correctly diagnose thyroid problems for treatment. Universal screening of thyroid problems pregnancy is currently not recommended, because large randomized controlled trials have not shown benefit of treating mild thyroid problems, which is more likely to be identified with universal screening. The findings of this study suggest a potential need for confirming mild thyroid problems in pregnancy with repeat testing. As women with TSH levels at either extreme (very low or very high) were more likely to still have abnormal levels on repeat measurements, it would be reasonable to start treatment in these women. Women with positive thyroid antibody levels would be also reasonable to treat, as they were likely to have persistent thyroid dysfunction on repeat measurements. However, those with only mild thyroid problems on one thyroid blood test may benefit from repeating them to confirm the diagnosis. In addition, studies regarding the impact of one-time abnormalities of thyroid levels on pregnancy outcomes are needed to assess long-term effects.

— Sun Y. Lee, MD, MSc

ABBREVIATIONS & DEFINITIONS

TSH: thyroid stimulating hormone — produced by the pituitary gland that regulates thyroid function; also the best screening test to determine if the thyroid is functioning normally.

Thyroxine (T4): the major hormone produced by the thyroid gland. T4 gets converted to the active hormone T3 in various tissues in the body.

TPO antibodies: these are antibodies that attack the thyroid instead of bacteria and viruses, they are a marker for autoimmune thyroid disease, which is the main underlying cause for hypothyroidism and hyperthyroidism in the United States.

Thyroglobulin antibodies: these are antibodies that attack the thyroid instead of bacteria and viruses, they are a marker for autoimmune thyroid disease, which is the main underlying cause for hypothyroidism and hyperthyroidism in the United States.

Antibodies: proteins that are produced by the body’s immune cells that attack and destroy bacteria and viruses that cause infections. Occasionally the antibodies get confused and attack the body’s own tissues, causing autoimmune disease.

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