Thyroid Disease and pregnancy

Overview and who to refer

Update (November 2024)

The management of with thyroid disease prior and during pregnancy can be complicated and thresholds for treatment and referral are subject to changes.

The current advice has been agreed and updated as of November 2024 and is being circulated to all relevant departments in BNSSG (i.e. gynaecology, endocrinology, obstetrics, midwifery, primary care, reporting laboratories). If you have previous documents saved elsewhere then these should be archived or deleted.

Who to Refer

Most patients with hypothyroidism prior and during pregnancy can be managed in the community using the guidelines below.

Patients who require referral should be referred to the appropriate antenatal clinic at UHBW or NBT (see Referral Section below) and will be managed in the Joint Endocrine Antenatal Clinic (JEANC).

Patients who are pregnant should be referred in the following scenarios:

• Patients with known hypothyroidism with a TSH>10mIU/L.
• Patients with a previous history of Graves’ disease or thyroid cancer or who have previously received radioactive iodine / undergone a thyroidectomy. 
• Patients with current hyperthyroidism.
• For women with TSH results persistently above the trimestral specific ranges, despite good compliance and a dose increase as per the chart below, please contact endocrinology for advice.

Patients without pre-existing thyroid disease

Screening Prior to Pregnancy

There is no national screening program for thyroid disease in pregnancy or prior to pregnancy.

However, it is advisable to consider screening for thyroid disease (TSH) in those in a high-risk group who are planning a pregnancy.

High risk groups include: 

• Previous positive TPO antibodies
• Personal history of thyroid disease
• First degree relative with thyroid disease
• Autoimmune disease
• Presence of a goitre
• Recurrent miscarriage (3 or more)
• Previous unexplained preterm delivery (<37 weeks)
• Taking lithium or amiodarone
• Previous thyroid surgery or head/neck irradiation

Thyroid Results in Pregnancy (in patients without pre-existing thyroid disease) 

In the setting of pregnancy, maternal hypothyroidism is defined as a TSH concentration above the upper limit of the trimestral specific range. It does not necessarily imply life-long hypothyroidism.

The normal reference ranges currently used in Bristol in each trimester for patients without pre-existing thyroid disease are as follows: 

Pre-conception management if no previous diagnosis of hypothyroidism

For patients with newly diagnosed hypothyroidism or subclinical hypothyroidism who are planning a pregnancy please consider the following advice:

Subclinical Hypothyroidism

Patients who are found to have sustained subclinical hypothyroidism (TSH>3.7mU/L on 2 consecutive occasions) and are planning a pregnancy should be commenced on 50mcg Levothyroxine.

They should be advised at the outset, that in the case of subclinical hypothyroidism, this does not imply life-long hypothyroidism and the need for Levothyroxine should be re-assessed post-delivery. If women are likely to be having further children, it may be advisable to continue until their family is complete. If they have completed their family, Levothyroxine should generally stop with review of their TSH 2-3 months’ later.

Overt Hypothyroidism

Patients with newly diagnosed overt hypothyroidism should be commence on Levothyroxine at a dose of 1.6mcg/kg to the nearest 25mcg.

Advice in assisted conception

Patients with positive TPO antibodies and a TSH in the normal range but ≥2.5mU/L may be treated in the context of assisted conception.

Pre-conception and pregnancy management for patients with pre-existing hypothyroidism

Pre-conception

For patients planning a pregnancy please see the following advice:

• There is no evidence that women with adequately treated hypothyroidism have any increased risk of obstetric complications.
• Inadequately treated hypothyroidism is associated with increased rate of early miscarriages and in early pregnancy can affect the neurocognitive development of babies. Patients should therefore understand the importance of adequate treatment at this time.
• In patients with hypothyroidism the goal pre- and during pregnancy is to maintain the TSH within the trimestral specific target ranges. Pre-pregnancy this would be aiming for a TSH between 0.38-2.5mU/L.

Pregnancy

Most patients with primary hypothyroidism can be managed in the community during their pregnancy using the tables below.

The treatment target ranges for the assay currently used in BNSSG are below: 

Unless thyroid function has been checked within the last 4 weeks, it is recommend to check TSH as soon as pregnancy is confirmed and increase the dose using recommendations below:

• Levothyroxine will usually need to be increased by 25-30%, but this varies between individual women (50-85% of women will require a dose increase).
• If TSH is >10mU/L then in addition to the dose increase, please refer these women urgently to the antenatal clinic (JEANC).
• If initial TSH <0.02mU/L this may reflect too much Levothyroxine but can also be normal in early pregnancy. If free T4 normal, simply keep on usual dose and repeat in 4 weeks. If free T4 is also high, please refer for advice from the endocrine antenatal clinic.
• Women diagnosed with hypothyroidism during pregnancy should be commenced on 50-100mcg Levothyroxine.
• Maternal TSH should be measured 4 weeks after any dose change. Once stable TSH should be tested every 6-8 weeks during first half of pregnancy, and again around 28 weeks.

Guide to adjusting Levothyroxine dose later in pregnancy

• Following delivery women should reduce their Levothyroxine back to their usual pre-pregnancy dose and have a blood test to check TSH at 6 weeks post-partum.
• Breast feeding is safe.

Medication Advice

The recommended treatment of maternal hypothyroidism is administration of oral Levothyroxine. Other thyroid preparations such as triiodothyronine (T3) or desiccated thyroid (Armour thyroid) should not be used in pregnancy. Please request endocrinology advice and guidance on how to convert to Levothyroxine.

Ideally, Levothyroxine should be taken first thing in the morning on an empty stomach. It should not be taken at the same time as other medications, particularly pregnancy multivitamins, as this can affect absorption. 

Referral

Most patients with hypothyroidism prior and during pregnancy can be managed in the community using the guidelines above.

Please see the 'Overview and who to refer' section at the top of the page for advice on which patients to refer.

Please see the Maternity Services page for advice on how to refer to antenatal clinics (including JEANC).

Resources

References: 

• https://www.btf-thyroid.org/pregnancy-and-thyroid-disorders-information-for-professionals
• Erik et al. 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid 2017; 2(3): 315-389.
• American College of Obstetricians and Gynecologists 2020 Thyroid disease in pregnancy: ACOG practice bulletin, number 223. Obstet Gynecol 135:e261–e274.

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