Thyroid tests and TSH targets

The approach to thyroid tests and TSH targets will differ depending on whether the woman has a thyroid condition, whether she is planning pregnancy, or whether she is already pregnant. The recommendation is that before and during pregnancy thyroid function tests are regulated such that serum TSH is kept in the lower half of the normal range (<2.5 mIU/l before and <2.0 during pregnancy). This may require a dose adjustment in your levothyroxine. Ideally during pregnancy thyroid function tests should be measured using trimester-specific reference ranges. 

Pregnancy and Thyroid Disease

Women with existing thyroid disease should let their doctor know they are trying for a baby because they may need to make changes to their medication.

Women with Hypothyroidism during Pregnancy

Women diagnosed with hypothyroid disease should be carefully monitored during pregnancy for thyroid hormone levels. It is especially critical during the first 12 weeks of pregnancy since the baby is dependent upon maternal thyroid hormones during this time. Ideally during pregnancy thyroid function tests should be measured using trimester-specific reference ranges. The recommendation is that during pregnancy thyroid function tests are regulated such that serum TSH is kept in the lower half of the normal range (<2.0). This may require a dose adjustment in your levothyroxine. In women already taking levothyroxine it is recommended that levothyroxine dosages be increased immediately by 25mcg-50mcg and monitored throughout the pregnancy with blood tests. 

Women with Hyperthyroidism during Pregnancy

Women with hyperthyroid disease should be carefully monitored during pregnancy. It is important that hormone levels remain in the normal range to ensure the baby develops normally. The free T4 (FT4) levels should be kept in the upper third of the pregnancy-specific reference range. The preferred anti-thyroid medication for pregnant women with hyperthyroidism is propylthiouracil in the first trimester and carbimazole for the rest of the pregnancy with close monitoring of thyroid hormone levels through blood tests.

Thyroid Disease occurring after Pregnancy

Women with no previously diagnosed thyroid disorder can develop postpartum thyroid disease for up to 12 months after the birth of the baby or even after miscarriage. About one in 12 women are diagnosed with postpartum thyroid disease, which is often temporary. The hypothyroidism (underactive thyroid) is treated with levothyroxine tablets. It is sometimes difficult to diagnose since many of the symptoms are those that come naturally to post-pregnancy motherhood like tiredness, irritability, weight gain, and mood changes. If any of these symptoms seem extreme and interfere with normal mothering activities, then see your doctor. A simple blood test can let you know.

Risks of Non Treatment of Thyroid Disease in Pregnancy

If a woman suffers from thyroid disease that is not detected before or during the pregnancy, there are risks to the mother and the baby. The pregnancy could be at risk for miscarriage during the early stages or developing pre-eclampsia, which causes high blood pressure. Placenta abruptio – detachment of the placenta – can also occur later in the pregnancy.

If thyroid disease is left untreated, there can be risks to the baby’s brain development during the pregnancy that may affect development in childhood.

Women with known Hypothyroidism planning a pregnancy

Women with hypothyroidism who wish to get pregnant should have their thyroxine dose adjusted to achieve a TSH of less than 2.5 mU/L or alternatively a TSH within the 1st trimester-specific reference range if such ranges are available locally. Lower targets may be indicated in women followed up for thyroid cancer but such targets should have already been decided by their endocrinologist or thyroid oncologist prior to conception.

Women undergoing evaluation for infertility

These women should have thyroid tests as part of the work up for infertility. If hypothyroidism (overt or subclinical) is detected this should be treated aiming for a TSH of less than 2.5 mU/L or TSH within the first trimester-specific reference range if such ranges are available locally.

Women without thyroid dysfunction and without infertility

These women do not routinely require thyroid tests either before or during pregnancy. However, they should be screened if they have a high risk of thyroid disease e.g. women with type 1 diabetes or other autoimmune disorders, women with a strong family history of thyroid disease, women with goitres, women on drugs known to cause thyroid dysfunction etc.

For further information

2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease during Pregnancy and the Postpartum

Management of Thyroid Dysfunction during Pregnancy and Postpartum: An Endocrine Society Clinical Practice Guideline

Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum

2014 European Thyroid Association Guidelines for the Management of Subclinical Hypothyroidism in Pregnancy and in Children

‘Thyroid function in pregnancy’ by Prof John Lazarus in the British Medical Bulletin, Volume 97 November 2010 

'Underactive Thyroid and Pregnancy' British Thyroid Association

'Thyroid disease in pregnancy' Wikipedia (submitted by British Thyroid Foundation)