What are the RCOG (Royal College of Obstetricians and Gynaecologists) and ACOG (American College of Obstetricians and Gynecologists) guidelines for managing thyroid disease in pregnancy?

RCOG and ACOG Guidelines for Thyroid Disease in Pregnancy

The management of thyroid disease during pregnancy should follow specific trimester-based protocols, with propylthiouracil (PTU) as the preferred antithyroid medication during the first trimester and methimazole recommended for the second and third trimesters. 1, 2

Hyperthyroidism in Pregnancy

Diagnosis and Initial Assessment

• Suppressed TSH with elevated T3 and T4 indicates hyperthyroidism, most commonly due to Graves' disease in pregnancy 1
• Rule out gestational transient thyrotoxicosis (associated with hyperemesis gravidarum), which rarely requires treatment 1

Treatment Algorithm

• First trimester: Use propylthiouracil (PTU) due to lower risk of congenital abnormalities 1, 2
• Second and third trimesters: Switch to methimazole 1, 2
• Goal: Maintain free T4 or Free Thyroxine Index (FTI) in the high-normal range using the lowest possible thioamide dosage 1, 2
• Monitor FT4 or FTI every 2-4 weeks to guide dosage adjustments 1, 2
• Check TSH level once each trimester 1

Symptom Management

• Beta-blockers (e.g., propranolol) can be used temporarily to control symptoms until thioamide therapy reduces thyroid hormone levels 1, 2

Potential Complications and Monitoring

• Monitor for side effects of thioamide therapy: agranulocytosis (presenting with sore throat and fever), hepatitis, vasculitis, and thrombocytopenia 1, 2
• Untreated maternal hyperthyroidism increases risks of severe preeclampsia, preterm delivery, heart failure, miscarriage, and low birth weight 1, 2

Alternative Treatments

• Thyroidectomy should be reserved only for women who do not respond to thioamide therapy or develop intolerance to antithyroid drugs 1, 2
• If surgery is necessary, the second trimester is the preferred timing 2
• Radioactive iodine (I-131) is absolutely contraindicated during pregnancy 1, 2

Hypothyroidism in Pregnancy

Screening and Diagnosis

• TSH is the primary test for diagnosing hypothyroidism in pregnancy 3
• Normal TSH values are lower during pregnancy than in non-pregnant women 3, 4
• Trimester-specific reference intervals should be used when available 4, 5
• When local reference ranges are not available, the upper cutoff limit should be approximately 4.0 mIU/L 5

Treatment Recommendations

• Pregnant women with elevated TSH should be treated with levothyroxine to restore TSH to the reference range 6
• Levothyroxine requirements often increase during pregnancy, necessitating dose adjustments 6
• Monitor serum TSH every 6-8 weeks during pregnancy and modify levothyroxine dose as needed 6

Special Considerations for Subclinical Hypothyroidism

• For women with subclinical hypothyroidism who are pregnant or planning pregnancy, levothyroxine treatment is recommended 6
• This recommendation is based on possible associations between high TSH and increased fetal wastage or neuropsychological complications in offspring 6
• For women already on levothyroxine with TSH in the subclinical hypothyroid range, assess compliance and appropriateness of dose 6

Postpartum Considerations

• Inform the newborn's physician about maternal thyroid disease due to risk of neonatal thyroid dysfunction 1, 2
• Women treated with PTU or methimazole can safely breastfeed 1
• Continue monitoring thyroid function postpartum, especially in women with positive thyroid antibodies who are at risk for postpartum thyroiditis 3

Thyroid Function Testing in Pregnancy

• TSH values are physiologically lower during pregnancy 3, 7
• Free T4 measurement is preferred over total T4 due to pregnancy-related changes in binding proteins 7
• TPOAb positivity occurs in 5.1-12.4% of women of childbearing age and may indicate increased risk for developing hypothyroidism during pregnancy 3

Common Pitfalls and Caveats

• Using non-pregnancy reference ranges for TSH may lead to misdiagnosis 4, 5
• Failure to monitor thyroid function regularly during pregnancy may result in inadequate treatment 6, 1
• Not switching from PTU to methimazole after the first trimester increases the risk of hepatotoxicity 1, 2
• Untreated or inadequately treated thyroid disease can have serious consequences for both mother and fetus 1, 2, 3