Management of Thyroid Disease in Pregnancy
Hyperthyroidism Management
Use propylthiouracil (PTU) exclusively during the first trimester, then switch to methimazole for the second and third trimesters to minimize both congenital malformations and maternal hepatotoxicity. 1, 2, 3
Medication Selection by Trimester
- First trimester (weeks 0-13): PTU is the preferred antithyroid medication due to lower risk of congenital abnormalities compared to methimazole 1, 2, 3
- Second and third trimesters: Switch to methimazole to reduce the risk of PTU-associated hepatotoxicity 2, 4
- The goal is to maintain free T4 or free thyroxine index (FTI) in the high-normal range using the lowest possible thioamide dosage 5, 1, 2
Monitoring Protocol
- Check free T4 or FTI every 2-4 weeks to adjust medication dosage 5, 1, 2, 3
- Once stable, check TSH every trimester 3
- Monitor for agranulocytosis (presents with sore throat and fever) - if these symptoms develop, obtain complete blood count immediately and discontinue the thioamide 5, 1
Adjunctive Therapy
- Beta-blockers (e.g., propranolol) can temporarily manage symptoms like tremors and palpitations until thioamide therapy reduces thyroid hormone levels 5, 1
- Monitor fetal heart rate and growth in women with Graves' disease 5, 3
Special Situations
- Hyperemesis gravidarum with biochemical hyperthyroidism: Rarely requires treatment unless other clinical signs of hyperthyroidism are present 1, 3
- Thyroid storm: Medical emergency requiring immediate treatment with PTU or methimazole, potassium/sodium iodide solutions, dexamethasone, phenobarbital, and supportive care without waiting for laboratory confirmation 1, 3
- Thyroidectomy: Reserved only for women who do not respond to thioamide therapy or develop severe drug intolerance (agranulocytosis, severe hepatotoxicity); if necessary, perform during second trimester 1, 3
Absolute Contraindications
- Radioactive iodine (I-131) is absolutely contraindicated during pregnancy as it causes fetal thyroid ablation 1, 3
- Women must wait four months after I-131 treatment before breastfeeding 1
Consequences of Inadequate Treatment
- Untreated or inadequately treated hyperthyroidism increases risks of severe preeclampsia, preterm delivery, heart failure, miscarriage, and low birth weight 2, 3
Hypothyroidism Management
Treat all pregnant women with elevated TSH using levothyroxine to restore TSH to trimester-specific reference ranges, as untreated hypothyroidism increases risks of preeclampsia, low birth weight, and neuropsychological defects in offspring. 5, 2
Dosing Strategy
- Pre-existing hypothyroidism: Increase levothyroxine dosage by 12.5 to 25 mcg per day as soon as pregnancy is confirmed, as requirements often increase by 30-50% during pregnancy 6, 7
- New onset hypothyroidism with TSH ≥10 IU/L: Start 1.6 mcg/kg/day 6
- New onset hypothyroidism with TSH <10 IU/L: Start 1.0 mcg/kg/day 6
Monitoring Protocol
- Measure serum TSH and free T4 as soon as pregnancy is confirmed 6
- Monitor TSH every 4 weeks until stable dose is reached and serum TSH is within normal trimester-specific range 6
- Continue monitoring every 6-8 weeks during pregnancy and modify levothyroxine dose as needed 2, 6
Target TSH Levels
- Maintain TSH ideally ≤2.5 mIU/L in the first trimester 7
- Maintain TSH ≤3.0 mIU/L in the second and third trimesters 7
Postpartum Management
- Reduce levothyroxine dosage to pre-pregnancy levels immediately after delivery 6
- Monitor serum TSH 4 to 8 weeks postpartum 6
Consequences of Inadequate Treatment
- Untreated maternal hypothyroidism increases risk of preeclampsia 5
- Maternal hypothyroidism from iodine deficiency increases risk of congenital cretinism (growth failure, mental retardation, neuropsychological defects) 5
- Inadequate treatment is associated with low birth weight in neonates 5
Postpartum Considerations
- Inform the newborn's physician about maternal thyroid disease (both hyperthyroidism and hypothyroidism) due to risk of neonatal thyroid dysfunction 5, 2, 3
- Women treated with PTU or methimazole can safely breastfeed 5, 2, 4
- Monitor for postpartum thyroiditis in women with history of thyroid dysfunction 3
Critical Pitfalls to Avoid
- Failing to switch from PTU to methimazole after first trimester increases risk of maternal hepatotoxicity 2
- Not increasing levothyroxine dose early in pregnancy for women with pre-existing hypothyroidism leads to inadequate treatment 6, 7
- Using radioactive iodine during pregnancy causes fetal thyroid ablation 1, 3
- Inadequate monitoring frequency (thyroid function should be checked every 2-4 weeks for hyperthyroidism and every 4 weeks for hypothyroidism until stable) 2, 6
- Treating biochemical hyperthyroidism associated with hyperemesis gravidarum when no other clinical signs are present is unnecessary 1, 3