Information Information for professionals Subclinical hypothyroidism in conception and pregnancy What is subclinical hypothyroidism? Subclinical hypothyroidism is defined as either having: A TSH greater than the upper limit of the non-pregnancy range and below 10 mU/L with normal FT4 before pregnancy. A TSH greater than the upper limit of the pregnancy reference range and below 10 mU/L during pregnancy. Please use the manufacturer-specific pregnancy reference ranges to make the diagnosis. What are the risks? If it is untreated, or inadequately treated, it may be harder for women to become pregnant, and their pregnancy may be at higher risk. These risks include premature birth, low birth weight, pregnancy loss, and problems with the baby’s brain development. How should it be treated? In women who are planning a pregnancy, or are pregnant, and who have subclinical hypothyroidism detected before the end of the 1st trimester, it is usually recommended to treat with levothyroxine. This is to reduce the risks of developing overt hypothyroidism and having associated complications during pregnancy. Before pregnancy Women’s care should be managed by their GP. Consider treatment with levothyroxine, particularly in women already known to be thyroid peroxidase antibody (TPOAb) positive, to achieve a preconception TSH ≤ 2.5 mU/L. When starting women on levothyroxine treatment for SCH, follow the guidance for women with overt hypothyroidism. During pregnancy If SCH is detected in the 1st trimester, start treatment with low dose or levothyroxine or consider retesting after 1-3 weeks to confirm diagnosis. If SCH is detected in the 2nd and 3rd trimesters, consider monitoring with thyroid function tests every 4-6 weeks until 20 weeks, then once again at 28 weeks for overt hypothyroidism. This is because treatment for SCH at this stage has not been shown to alter outcomes. After birth Advise to stop taking levothyroxine if they were not taking it before their pregnancy. Check thyroid function, usually during 6-week postnatal check. Read the RCOG Green-top guideline on the Management of Thyroid Disorders in Pregnancy Manage Cookie Preferences Please ensure Javascript is enabled for purposes of website accessibility