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THYROID AND PREGNANCY
How long do pregnancy-related thyroid problems last?
BACKGROUND
Disorders of the thyroid, including autoimmune disorders, can develop during pregnancy. These disorders may lead to problems in the mother and the baby. It is well known that thyroid hormone levels change during pregnancy and some of these changes may be interpreted as abnormal. This includes isolated low T4 levels (hypothyroxinemia), gestational thyrotoxicosis and overt hyperthyroidism. It is recognized that test results of thyroid function vary in pregnancy based on the pregnancy trimester.
This study aims to analyze thyroid function changes in different trimesters of pregnancy and compare these changes with pregnancy outcomes.
THE FULL ARTICLE TITLE
Yang Z, et al. The dynamic changes in maternal thyroid parameters across the three trimesters and their differential effects on the occurrence of adverse obstetric outcomes Endocr Metab Immune Disord Drug Targets. Epub 2025 Mar 4; PMID: 40045850.
SUMMARY OF THE STUDY
This study included 390 pregnant Chinese women between 2018 and 2022 from the Maternal-Fetal Cohort in Northeast China. The included participants were evaluated and had blood samples drawn to measure thyroid function tests through each trimester and after delivery.
The prevalence of thyroid dysfunction in the first, second, and third trimesters were, respectively, 0.3%, 2.0%, and 0.3% for subclinical hypothyroidism; 7.0%, 2.9%, and 2.0% for isolated hypothyroxinemia; 1.7%, 2.4% and 3.8% for subclinical hyperthyroidism, 2.9%, 0%, and 0% for overt hyperthyroidism (e.g., thyrotoxicosis); and 0% in all trimesters for overt hypothyroidism.
The study showed that 1 out of 2 patients with isolated hypothyroxinemia in the first trimester improved to normal in the second trimester, while 2 out of 3 of these patients were normal in the third trimester. For those with overt hyperthyroidism in the first trimester, 7 of 10 were normal in the 2nd trimester, and the remaining 3 had levels improve to the level of subclinical hyperthyroidism in the second trimester. Serum FT4 and FT3 levels declined in all patients, while TSH levels increased from first to third trimester of pregnancy. Thyroid antibody levels decreased with time in those with thyroid autoimmunity.
In terms of outcomes, high blood pressure during pregnancy was higher in patients with thyroid autoimmunity and isolated hypothyroxinemia in the first trimester. Subclinical hypothyroidism was associated with postpartum hemorrhage, and subclinical thyrotoxicosis was associated with small for gestational age infants.
WHAT ARE THE IMPLICATIONS OF THIS STUDY?
The study shows that thyroid problems can resolve spontaneously as pregnancy progresses, indicating the necessity of repeated testing during different trimesters of pregnancy. While most of the thyroid problems do not require treatment, it is clear that some of these changes may have adverse outcomes during pregnancy. Finally, the ongoing questions as to what disorders are treated and which are simply watched are still unclear. Hence, it is worth further evaluation in monitoring thyroid function tests in pregnant patients presenting with thyroid dysfunction.
— Joanna Miragaya
ATA RESOURCES
Thyroid Disease in Pregnancy: https://www.thyroid.org/thyroid-disease-pregnancy/
Thyroid Function Tests: https://www.thyroid.org/thyroid-function-tests/
ABBREVIATIONS & DEFINITIONS
Autoimmune thyroid disease: a group of disorders that are caused by antibodies that get confused and attack the thyroid. These antibodies can either turn on the thyroid (Graves’ disease, hyperthyroidism) or turn it off (Hashimoto’s thyroiditis, hypothyroidism).
Hypothyroidism: a condition where the thyroid gland is underactive and doesn’t produce enough thyroid hormone. Treatment requires taking thyroid hormone pills.
Subclinical Hypothyroidism: a mild form of hypothyroidism where the only abnormal hormone level is an increased TSH. There is controversy as to whether this should be treated or not.
Overt Hypothyroidism: clear hypothyroidism an increased TSH and a decreased T4 level. All patients with overt hypothyroidism are usually treated with thyroid hormone pills.
Primary hypothyroidism: the most common cause of hypothyroidism cause by failure of the thyroid grand.
Transient hypothyroxinemia: temporary decrease in the blood level of thyroxine (T4) after delivery in pre-term infants, followed by the return of normal levels in the absence of any treatment.
Hyperthyroidism: a condition where the thyroid gland is overactive and produces too much thyroid hormone. Hyperthyroidism may be treated with antithyroid meds (Methimazole, Propylthiouracil), radioactive iodine or surgery.
Subclinical Hyperthyroidism: a mild form of hyperthyroidism where the only abnormal hormone level is a decreased TSH.
Thyroxine (T4): the major hormone produced by the thyroid gland. T4 gets converted to the active hormone T3 in various tissues in the body.
Triiodothyronine (T3): the active thyroid hormone, usually produced from thyroxine.
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.
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