Can hypothyroidism (underactive thyroid) occur during pregnancy?

Hypothyroidism in Pregnancy

Yes, hypothyroidism can and does occur during pregnancy, with significant implications for both maternal and fetal health if left untreated. Hypothyroidism affects approximately 1 in 1600-2000 pregnancies according to recent publications 1, with prevalence of undiagnosed subclinical hypothyroidism ranging from 3% to 15% in pregnant women 2.

Causes of Hypothyroidism in Pregnancy

• Primary causes:
  • Hashimoto's disease (chronic autoimmune thyroiditis) - most common cause in industrialized nations 3
  • Iodine deficiency - most common cause worldwide 3
  • Previous thyroid treatments (radioactive iodine therapy, thyroidectomy) 3
  • Subacute thyroiditis 3

Clinical Presentation

Hypothyroidism during pregnancy may present with:

• Fatigue
• Muscle cramps
• Constipation
• Cold intolerance
• Hair loss
• Weight gain
• Intellectual slowness
• Voice changes
• Insomnia 3

However, diagnosis based solely on clinical grounds can be difficult, even in advanced cases, requiring laboratory confirmation 1.

Maternal and Fetal Risks

Untreated maternal hypothyroidism is associated with significant adverse outcomes:

Maternal risks:

• Increased risk of preeclampsia 3
• Preterm birth 3, 2
• Placental abruption 3
• Gestational hypertension 2
• Gestational diabetes 2
• Miscarriage 2, 1

Fetal/neonatal risks:

• Fetal death 3
• Low birth weight 3
• Cognitive impairment in children 3
• Congenital cretinism (with iodine deficiency) 3
• Decreased IQ in offspring 2

Diagnosis

The recommended diagnostic approach includes:

1. TSH testing - primary screening test for thyroid dysfunction 3, 4
2. Free T4 testing - should be performed alongside TSH in suspected hypothyroidism 3, 4
3. TPO antibody testing - indicates autoimmune etiology and predicts higher risk of developing overt hypothyroidism 4

Normal TSH ranges during pregnancy are lower than non-pregnant values:

• First trimester: upper limit 2.5 mIU/L
• Second and third trimesters: upper limit 3.0 mIU/L 2

Treatment

Levothyroxine is the treatment of choice for hypothyroidism in pregnancy 1, 5. The goal is to normalize thyroid function as quickly as possible to reduce risks.

Treatment recommendations:

1. Overt hypothyroidism (elevated TSH with low free T4):

   • Initiate levothyroxine immediately 4, 5
   • Starting dose: 1.6 mcg/kg/day 5

2. Subclinical hypothyroidism (elevated TSH with normal free T4):

   • Initiate levothyroxine for TSH >10 mIU/L 4
   • Also recommended for pregnant women with any TSH elevation 4

3. Dosage adjustments:

   • Pregnancy typically increases levothyroxine requirements by 30% or more by 4-6 weeks gestation 3, 5
   • Monitor TSH every 4 weeks until stable dose is reached 5
   • Target TSH: trimester-specific reference range 5

4. Post-delivery:

   • Return to pre-pregnancy dosage immediately after delivery 5
   • Monitor TSH 4-8 weeks postpartum 5, 1

Monitoring

• Measure TSH and free T4 every 4 weeks during first half of pregnancy 5
• Adjust dosage as needed to maintain TSH in trimester-specific reference range 5
• Continue monitoring throughout pregnancy 3

Screening Recommendations

Women at high risk for developing hypothyroidism should be screened:

• Previous treatment for hyperthyroidism
• High-dose neck irradiation
• Evidence of thyroid autoimmunity
• Type 1 diabetes
• History of miscarriage or preterm delivery 1

While universal screening remains controversial, targeted case finding is generally practiced, though recent evidence suggests universal screening might be beneficial 6.

Key Clinical Considerations

1. Early detection and treatment are essential to prevent adverse outcomes 6
2. Subclinical hypothyroidism should be treated during pregnancy to prevent complications 2
3. TPO antibody positivity even with normal thyroid function may increase miscarriage risk and lead to postpartum thyroiditis 1
4. Post-partum follow-up is mandatory as women may have a flare-up of autoimmune thyroid disease or continue to require replacement therapy 6

By promptly identifying and treating hypothyroidism during pregnancy, most adverse maternal and fetal outcomes can be prevented, making awareness and appropriate management of this condition crucial for optimal pregnancy outcomes.