New-onset or previously untreated hypothyroidism in pregnancy

Pregnancy can precipitate hypothyroidism in women with risk factors for thyroid disease, or exacerbate pre-existing subclinical disease, because the body cannot meet the increased demand for thyroid hormone that occurs during pregnancy.

New-onset hypothyroidism during pregnancy should be managed with specialist input.

If overt primary hypothyroidism is newly diagnosed in a pregnant woman, start thyroxine replacement therapy immediately. The harms of untreated overt hypothyroidism in pregnancy are well established.

If subclinical hypothyroidism is newly diagnosed in a pregnant woman, or if the serum TSH concentration is at the upper end of the normal range, test for thyroid peroxidase antibody. Thyroid peroxidase antibody–positivity may increase the risk of pregnancy complications, including in women with a TSH concentration within the normal range.

For subclinical hypothyroidism, start levothyroxine treatment if the serum TSH concentration is:

• above the reference range and thyroid peroxidase antibody is positive
• above 10 milliunits/L regardless of thyroid peroxidase antibody–positivity.

The evidence to support treatment at lower thresholds is less clear; however, the risk of harm with levothyroxine therapy is low, so treatment can also be considered if the serum TSH concentration is:

• at the upper end of the normal range and thyroid peroxidase antibody is positive
• above the reference range but below 10 milliunits/L and thyroid peroxidase antibody is negative.

After delivery, monitor the serum TSH concentration—levothyroxine therapy can usually stopped.

Women with hypothyroidism during pregnancy are at increased risk of Postpartum thyroid dysfunction.