How is hypothyroidism (underactive thyroid) managed in pregnant women to minimize adverse maternal outcomes?

Management of Hypothyroidism in Pregnancy to Minimize Maternal Outcomes

Pregnant women with hypothyroidism should be treated immediately with levothyroxine, increasing the dose by approximately 30% as soon as pregnancy is confirmed, with TSH monitoring every 4 weeks during the first half of pregnancy and every trimester thereafter to maintain TSH in the normal range and prevent maternal complications including preeclampsia, gestational hypertension, spontaneous abortion, and stillbirth. 1

Immediate Treatment Initiation

• Levothyroxine should never be discontinued during pregnancy, and hypothyroidism diagnosed during pregnancy must be promptly treated to prevent adverse maternal outcomes 1
• Untreated maternal hypothyroidism is associated with spontaneous abortion, gestational hypertension, preeclampsia, stillbirth, and premature delivery 1
• The FDA explicitly states that levothyroxine should not be discontinued during pregnancy based on clinical experience showing no increased rates of major birth defects or adverse maternal outcomes with treatment 1

Proactive Dose Adjustment Strategy

For women with pre-existing hypothyroidism planning pregnancy:

• Increase levothyroxine dose by 30% immediately upon pregnancy confirmation, as requirements increase as early as the fifth week of gestation 2
• This proactive approach prevents the biochemical hypothyroidism that occurs in many pregnant women despite being on treatment 2
• Levothyroxine requirements increase by 47% on average during the first half of pregnancy, with median onset at 8 weeks gestation and plateau by week 16 2

TSH Targets and Monitoring Schedule

Optimal preconception TSH management:

• Women planning pregnancy should have TSH maintained below 1.2 mIU/L preconception, as 50% of women with TSH 1.2-2.4 mIU/L require dose increases during pregnancy compared to only 17.2% with TSH <1.2 mIU/L 3
• This lower target minimizes the risk of inadequate treatment during critical early fetal development 3

Monitoring frequency:

• Check TSH every 4 weeks during dose titration to maintain TSH in the normal range 4
• Once stable, evaluate thyroid function every trimester 4
• The goal is maintaining free T4 in the high-normal range using the lowest possible medication dose 5

Etiology-Specific Dose Requirements

The magnitude of dose increases varies substantially by underlying cause 6:

• Primary hypothyroidism: Requires modest increases (11% first trimester, 16% second and third trimesters) from baseline dose averaging 92.5 mcg daily 6
• Treated Graves' disease or goiter: Requires the largest increases (27% first trimester, 51% second trimester, 45% third trimester) from baseline dose averaging 140.4 mcg daily 6
• Thyroid cancer: Requires moderate increases (9% first trimester, 21% second trimester, 26% third trimester) from baseline dose averaging 153.2 mcg daily 6

Maternal Complications of Inadequate Treatment

Specific adverse maternal outcomes from untreated or inadequately treated hypothyroidism:

• Preeclampsia risk is significantly elevated with untreated hypothyroidism 5, 1
• Gestational hypertension occurs at higher rates 1
• Spontaneous abortion risk increases 1
• Stillbirth and premature delivery are more common 1
• Inadequate treatment specifically is associated with low birth weight 5

Subclinical Hypothyroidism Management

• Subclinical hypothyroidism (elevated TSH with normal T4) during pregnancy is associated with increased risks of preeclampsia, low birth weight, and impaired fetal neuropsychological development 5
• Treatment is recommended for subclinical hypothyroidism in pregnancy, though professional groups differ on universal screening recommendations 7
• TSH >10 mIU/L warrants treatment regardless of symptoms due to high progression risk and maternal complications 5, 8

Postpartum Dose Adjustment

• Return levothyroxine dose to pre-pregnancy level immediately after delivery, as postpartum TSH levels return to preconception values 1
• Reassess thyroid function 6-12 weeks postpartum 9

Critical Pitfalls to Avoid

• Never delay treatment initiation while awaiting repeat testing in pregnant women with elevated TSH, as fetal development is time-sensitive 1
• Avoid waiting for symptoms to develop before increasing doses, as biochemical hypothyroidism can occur despite stable pre-pregnancy dosing 2
• Do not use standard non-pregnancy TSH targets, as pregnancy-specific reference ranges are lower (TSH <2.5 mIU/L first trimester, <3.0 mIU/L second and third trimesters) 7
• Failing to anticipate dose increases based on etiology leads to inadequate treatment during critical developmental windows 6