PLEASE NOTE UPDATES IN THE REFERENCE RANGES: THESE ARE TRIMESTER-SPECIFIC
Early diagnosis and good management of maternal thyroid dysfunction are essential to ensure minimal adverse effects on foetal development and maternal health. The following are suggestions for using thyroid function tests in Primary Care and are derived from the UK guidelines, modified to account for local practice.
Key safety messages are highlighted in red – especially that anyone with established hypothyroidism should immediately increase the T4 dose by 25 micrograms as soon as a pregnancy is confirmed.
The guidance covers:
Diagnosis and Management of thyroid disease in pregnancy
- The reference range for TFTs in pregnancy
- Patients with detectable TRAbs.
- Assessing hypothyroidism in pregnancy
- Patients with a history of Grave’s disease (whether or not hypothyroid)
- Patients diagnosed with hypothyroidism during pregnancy
- Hypothyroidism: key safety points
- Hypothyroidism in pregnancy – flow diagram.
Sub-Clinical hypothyroidism and pregnancy
- Subclinical hypothyroidism in pregnancy: background
- Subclinical hypothyroidism in pregnancy: assessment and treatment algorithm
- Sub-clinical hypothyroidism in reproductive age women – flow diagram
- Planning pregnancy in hyperthyroidism
- TFTs and hyperthyroidism in pregnancy – TRAb is crucial
- Hyperthyroidism: key safety points
- Hyperthyroidism in pregnancy – flow diagram
- Patients with detectable TRAbs
- Specialist management of hyperthyroidism in pregnancy
- CBZ/PTU therapy: postnatal management.
Diagnosis and Management of Thyroid Disease in Pregnancy
This requires close liaison between the GP, community midwife, endocrinologist and obstetrician: much of the thyroid function testing is likely to be undertaken by the community midwives. However, the initial set of thyroid function tests requested for screening purposes or to check thyroid status in patients with established thyroid disorders is more likely to be done by the GP. For the majority of stable hypothyroid patients on Levothyroxine, NHS Lothian guidance suggests that 2 yearly testing is adequate, but in women of reproductive age, TFTs should be done at least annually.
THE REFERENCE RANGE FOR TFTs IN PREGNANCY.
Maternal Free T4 (FT4) and Free T3 (FT3) rather than total hormone concentrations must be measured in pregnancy. This is because Total T4 and Total T3 increase in pregnancy due to increased serum concentrations of thyroid hormone binding proteins. It is only the FT3 and FT4 fraction (not the bound fraction) that can enter cells and modify metabolism. Trimester-specific reference ranges for FT3 and FT4 need to be applied for diagnosis as their concentrations fall during pregnancy (see below).
| 1st trimester | 2nd trimester | 3rd trimester | |
| FT4 pmol/l | 10-28 | 9-28 | 8-28 |
| FT3 pmol/l | 3.8-6.0 | 3.2-5.5 | 3.1-5.0 |
| TSH mU/l | 0.33-4.59 | 0.35-4.10 | 0.21-3.15 |
PLEASE SEE THE INDIVIDUAL CONDITIONS PAGES FOR MORE SPECIFIC TFT TESTING AND MANAGEMENT ADVICE.
Patients with detectable TRAbs
- The Endocrinologist and Obstetrician should be informed of any patient with detectable TRAbs.
- Women with detectable TRAb should be advised to deliver in hospital.
- Further TRAb measurements and ultrasound scans will be required, the frequency of which will be advised by the Endocrinologist and Obstetrician. Typically a scan will be required each trimester, in addition to growth scans at 28 and 34 weeks (more often if control is poor).
- Paediatricians should be informed of delivery within the first 12 hours of life and the consultant obstetrician will document this in the neonatal management plan during the antenatal period. The infant should be seen within the first 24 hours of life if TRAb are detectable at 36 weeks or if the TFTs from cord blood are abnormal
- Cord blood should be taken for TSH, FT4 and Total T3 at delivery and the baby should have a resting heart rate checked and remain in hospital for at least 24 hours. Further repeat TSH, Free T4 and total T3 in the neonate should be carried out on the advice of the neonatal team.
REFERENCES – please see Resources and Links
C.M. & N.Z. 08-07-24
Who to refer:
- ALL women with hyperthyroidism
- Patients with a history of Graves’ Disease who are euthyroid or hypothyroid through radioiodine treatment or surgery – who are TRAbs positive
- Any woman with unstable TFTs
- TPO Ab-positive women: management is covered in the section on sub-clinical hypothyroidism. These women do not necessarily need referral, but if advice is required, please discuss with a consultant endocrinologist by sending an ‘Advice Only’ referral on SCI Gateway
- Community midwife to inform obstetrician if hypothyroidism diagnosed during pregnancy
- All women with hyperthyroid profile post-partum to assess for post-partum thyroiditis and Graves’ disease – refer to Endocrinology
- Any woman with a HISTORY of hyperthyroidism should also be discussed or referred
- All women with hyperthyroidism in pregnancy should be seen by a Consultant Endocrinologist and a Consultant Obstetrician from early in pregnancy – please see Hyperthyroidism in Pregnancy section for full details.
1. Toft A. 2004 Increased levothyroxine requirements in pregnancy- Why, when and how much? NEJM;351(3): 292-3.
2. Alexander EK, Marqusee E, Lawrence J et al. Timing and magnitude of increases in levothyroxine requirements during pregnancy in women with hypothyroidism. NEJM 2004; 351:241-9.
3. Demer LM, Spencer CA. Laboratory medicine practice guidelines: laboratory support for the diagnosis and monitoring of thyroid disease. Clin Endocrinol 2003; 58:138-40.
4. Pop VJ, Brouwers EP, Vader HL et al. Maternal hypothyroxinaemia during early pregnancy and subsequent child development: a 3-year follow-up study. Clin Endocrinol 2003;59 :282-8.
5. Haddow JE, Palomaki GE, Allan WC et al. Maternal thyroid deficiency during pregnancy and subsequent neuropsychological development of the child. NEJM 1999; 341: 549-55.
6. Luton D, Le Gac I, Vuillard E et al. Management of Grave’s disease during pregnancy: the key role of foetal thyroid gland monitoring. J Clin Endocrinol and Metabolism 2005;90: 6093.
7. Harborne LR, Alexander CE, Thomson AJ, O’Reilly DS, Greer IA. Outcomes of pregnancy complicated by thyroid disease. Aust NZ J Obstet Gynaecol 2005; 45(3): 239-242.
8. Thyroid disorders in pregnancy. Curr Opin Obstet Gynecol 2005;17(2):123-7.
9. Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid: 27(3), 2017: 315-389. https://www.liebertpub.com/doi/pdfplus/10.1089/thy.2016.0457
10. Dhillon-Smith et al. Levothyroxine in women with Thyroid Peroxidase antibodies before conception. New England Journal of Medicine 2019; 380:1316-1325.
