What is the recommended evaluation and management of overt and subclinical hypothyroidism and hyperthyroidism in a pregnant woman, including appropriate laboratory testing, trimester‑specific treatment dosing, monitoring intervals, and postpartum care?

Management of Thyroid Disorders in Pregnancy

All pregnant women with overt hypothyroidism require immediate levothyroxine treatment targeting TSH <2.5 mIU/L in the first trimester, while hyperthyroidism requires propylthiouracil in the first trimester followed by methimazole thereafter, as untreated thyroid dysfunction causes preeclampsia, preterm delivery, fetal death, and permanent neurocognitive impairment in offspring. 1, 2

Initial Laboratory Evaluation

Diagnostic Testing:

• TSH is the initial screening test for all pregnant women with suspected thyroid dysfunction 3
• Both TSH and free T4 (FT4) or free thyroxine index (FTI) must be obtained when hyperthyroidism or hypothyroidism is suspected 3
• Antibody testing depends on the clinical scenario, particularly TSH receptor antibodies in women with current or prior Graves' disease 3

Overt Hypothyroidism Management

Immediate Treatment Protocol:

• Start levothyroxine immediately upon diagnosis without waiting for repeat testing, as first-trimester hypothyroidism specifically causes cognitive impairment in children 1
• Target TSH <2.5 mIU/L in the first trimester 1
• Maintain free T4 in the high-normal range throughout pregnancy 1

Dosing and Monitoring:

• Women already on levothyroxine should increase their dose by 25-50% immediately upon pregnancy confirmation 1
• Adjust levothyroxine by 12.5-25 mcg increments based on TSH results 1
• Check TSH every 4 weeks until stable, then every trimester 3
• Most women require 25-50% dose increase above pre-pregnancy levels 1

Critical Maternal and Fetal Risks:

• Untreated hypothyroidism increases risk of preeclampsia, low birth weight, placental abruption, and fetal death 3, 1
• Inadequate treatment causes permanent neurocognitive impairment in offspring 1
• Congenital cretinism (growth failure, mental retardation, neuropsychologic defects) occurs with severe iodine deficiency 3

Subclinical Hypothyroidism

Treatment Approach:

• Evidence for treating subclinical hypothyroidism remains controversial, with major trials showing no clear cognitive benefit but potential reduction in obstetric complications 4, 5
• Women with elevated TSH and positive TPO antibodies have increased risk of miscarriage and preterm delivery 6
• Treatment with levothyroxine in antibody-positive women reduces preterm birth risk (RR: 0.31) 5
• Given the minimal risk of levothyroxine and potential for harm from untreated disease, treat subclinical hypothyroidism targeting TSH <2.5 mIU/L in the first trimester, particularly in antibody-positive women 1, 7

Overt Hyperthyroidism Management

First-Line Medical Treatment:

• Propylthiouracil (PTU) is the preferred antithyroid medication during the first trimester due to lower risk of congenital abnormalities compared to methimazole 2
• Switch to methimazole in the second and third trimesters 2
• Target free T4 or FTI in the high-normal range using the lowest possible thioamide dosage 3, 2

Monitoring Protocol:

• Measure free T4 or FTI every 2-4 weeks to guide dosage adjustments 3, 2
• Check TSH every trimester once stable 3
• Monitor for agranulocytosis (sore throat, fever) - obtain complete blood count and discontinue thioamide immediately if suspected 3
• Other side effects include hepatitis, vasculitis, and thrombocytopenia 3

Adjunctive Therapy:

• Beta-blockers (propranolol) can temporarily manage symptoms (tremors, tachycardia, palpitations) until thioamide reduces thyroid hormone levels 3, 2

Maternal and Fetal Risks:

• Inadequately treated hyperthyroidism increases risk of severe preeclampsia, preterm delivery, heart failure, miscarriage, and low birth weight 3
• Graves' disease causes 95% of hyperthyroidism cases in pregnancy 3
• Fetal thyrotoxicosis and neonatal thyroid dysfunction can occur due to transplacental antibody passage 3

Special Clinical Scenarios

Graves' Disease Considerations:

• Diagnosis requires elevated FT4 or FTI with suppressed TSH, without thyroid mass or nodular goiter 3
• Distinctive signs include eyelid lag/retraction and pretibial myxedema 3
• Monitor fetal heart rate and growth; ultrasound screening for fetal goiter only if problems detected 3
• Inform the newborn's physician about maternal Graves' disease due to neonatal thyroid dysfunction risk 3, 2
• Transient fetal/neonatal thyroid suppression from thioamide therapy usually requires no treatment 3

Thyroid Storm:

• Medical emergency characterized by fever, disproportionate tachycardia, altered mental status, vomiting, diarrhea, and cardiac arrhythmia 2
• Treat immediately without waiting for laboratory confirmation with PTU or methimazole, potassium/sodium iodide, dexamethasone, phenobarbital, and supportive care 2
• Avoid delivery during thyroid storm unless absolutely necessary 2

Hyperemesis Gravidarum:

• Biochemical hyperthyroidism associated with hyperemesis rarely requires treatment unless other clinical hyperthyroid signs are present 2
• Routine thyroid testing not recommended unless other hyperthyroid signs exist 2

Surgical Management:

• Thyroidectomy reserved for women who do not respond to thioamide therapy or develop severe drug intolerance (agranulocytosis, severe hepatotoxicity) 3, 2
• Second trimester is the preferred timing if surgery necessary 2

Absolute Contraindications:

• Radioactive iodine (I-131) is absolutely contraindicated in pregnancy as it causes fetal thyroid ablation 3, 2
• If inadvertent exposure occurred before 10 weeks gestation, fetal thyroid unlikely ablated; after 10 weeks, consider risk of congenital hypothyroidism 3
• Women must wait 4 months after I-131 treatment before breastfeeding 3, 2

Postpartum Care

Breastfeeding:

• Women treated with propylthiouracil or methimazole can breastfeed safely 3

Monitoring:

• Women with positive TPO antibodies have increased risk of postpartum thyroiditis 6
• Selenium supplementation during and after pregnancy may reduce postpartum thyroiditis risk 6
• Graves' disease typically improves during pregnancy but recurs after delivery 6

Critical Pitfalls to Avoid

• Never delay treatment waiting for repeat testing - fetal harm occurs before maternal symptoms appear 1
• Never target TSH >2.5 mIU/L in the first trimester - even subclinical hypothyroidism associates with adverse pregnancy outcomes 1
• Never use radioactive iodine during pregnancy - it causes fetal thyroid ablation 3, 2
• Never continue methimazole in the first trimester when PTU is available - methimazole has higher teratogenic risk 2
• Never ignore sore throat and fever in women on thioamides - immediately check complete blood count and discontinue medication if agranulocytosis suspected 3