According to the 2011 "Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum," the typical Thyroid Stimulating Hormone (TSH) reference range for women who are not pregnant has an upper range of around 4.0 mIU/L. The Guidelines recommend, however, that the upper reference range during pregnancy should be approximately 2.5 to 3.0 mIU/L.
Specifically, the Guidelines recommend that if a laboratory has not established its own trimester-specific reference ranges for TSH, the following reference ranges should be used:
- First trimester: 0.1-2.5 mIU/L
- Second trimester: 0.2-3.0 mIU/L
- Third trimester: 0.3-3.0 mIU/L
In pregnancy, overt hypothyroidism is defined as a TSH above 2.5 mIU/L, along with a decreased Free Thyroxine (Free T4) level. Even if a woman has normal Free T4, if TSH is above 10.0 mIU/L during pregnancy, it is also considered to be overt hypothyroidism. Subclinical hypothyroidism is defined as TSH between 2.5 and 10 mIU/L, with a normal Free T4 level.
In women who are not iodine-deficient, hypothyroidism is most commonly caused by autoimmune Hashimoto's disease. According to the Guidelines, the thyroid peroxidase antibodies (TPOAb) characteristic of Hashimoto's disease were detected in about half of pregnant women with subclinical hypothyroidism, and in more than 80% of women with overt hypothyroidism.
Subclinical Hypothyroidism in PregnancyAccording to the Guidelines, subclinical hypothyroidism increases pregnancy complications. In particular, there is a significantly higher rate of miscarriage (6.1%) in women with Hashimoto's disease who have TSH levels between 2.5 and 5.0 mIU/L, versus the 3.6% miscarriage rate of women who have Hashimoto's disease with TSH levels below 2.5 mIU/L.
Overt Hypothyroidism in PregnancyAccording to the Guidelines, overt hypothyroidism should be treated in pregnancy. This includes women with TSH levels above the trimester-specific ranges with decreased Free T4, and all pregnant women with a TSH above 10, even if Free T4 is normal.
According to the Guidelines, the recommended treatment for a pregnant woman with hypothyroidism is oral levothyroxine. According to the Guidelines, it is "strongly recommended not to use other thyroid preparations, such as T3 or natural desiccated thyroid drugs."
The Guidelines do not include any citations or explanation regarding this recommendation, but in an email interview, the Guidelines' lead author, Alex Stagnaro-Green, MD, cited two studies from 1993 and 1994 that have shown that "T4 can cross the placenta, but levels of T3 that cross are very low, therefore T3 only, combined T4/T3 and desiccated thyroid are all not recommended for women attempting pregnancy or who are pregnant."
Managing Treated Hypothyroidism During PregnancyThe Guidelines indicate that a pregnant woman's total T4 concentrations need to increase by 20% to 50% in order to meet the demands of pregnancy. This happens naturally in a woman with healthy thyroid function. However, in a woman who is being treated for hypothyroidism and becomes pregnant, the thyroid is unable to respond to the hormonal cues to increase T4 production, resulting in a need for an increased dosage of thyroid hormone replacement medication.
According to the Guidelines, the increased demand begins as early as weeks 4 to 6 of pregnancy, and typically increases until weeks 16 to 20, when it plateaus until delivery. From 50% to 80% of hypothyroid women who are being treated with levothyroxine will need to increase their dose during pregnancy.
The increase in levothyroxine, when needed, should come as soon as the pregnancy is confirmed. The Guidelines state that for women who are euthyroid, and taking a once-daily dose of levothyroxine, one option to ensure optimal dosing is for the woman to add two tablets per week -- a 29% increase in dosage -- as soon as a period is missed or pregnancy is suspected.
The Guidelines also recommend that a woman with hypothyroidism have her dosage adjusted so that TSH is below 2.5 mIU/L prior to conception. This lowers the risk of the TSH elevating in the first trimester.
Women who are hypothyroid and become pregnant should have thyroid tests run every four weeks during the first half of pregnancy, in order to allow for dosage adjustments. The TSH should again be checked between weeks 26 and 32 of pregnancy. The Guidelines recommend that following delivery, the dosage be reduced to the pre-pregnancy dose, and rechecked at six weeks post delivery.
The Guidelines do note, however, that some studies show that more than half the women who have Hashimoto's disease and are on thyroid hormone replacement actually have ended up with a postpartum increase over the pre-pregnancy dose, likely because pregnancy worsens autoimmune thyroid dysfunction.
Hashimoto's Disease During PregnancyThe Guidelines recommend that women with Hashimoto's who are not receiving thyroid treatment be monitored every 4 to 6 weeks during pregnancy until mid-pregnancy, and any subclinical or hypothyroidism -- per the trimester-specific ranges -- be treated. The pregnant woman should be tested at least once between weeks 26 and 32 of pregnancy.
Burrow G, et. al. "Maternal and fetal thyroid function." New England Journal of Medicine. 1994;331:1072-8
Contempré B, et. al. "Detection of thyroid hormones in human embryonic cavities during the first trimester of pregnancy." Journal of Clinical Endocrinology and Metabolism. 1993;77(6):1719-22.
Stagnaro-Green, Alex, et. al. "Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum." Thyroid. Volume 21, Number 10, 2011 (Online)