Thyroid Conditions and Pregnancy

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.

Hypothyroidism in pregnancy

• 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

Hyperthyroidism and pregnancy

• 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.

Post-partum thyroiditis.

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 EACH SECTION BELOW FOR MORE SPECIFIC TFT ADVICE RELATING TO INDIVIDUAL CONDITIONS.

1. Patients with detectable TRAbs
2. The Endocrinologist and Obstetrician should be informed of any patient with detectable TRAbs.
3. Women with detectable TRAb should be advised to deliver in hospital.
4. 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).
5. 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
6. 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.