What are the primary causes of hypothyroidism in pregnancy?

Primary Causes of Hypothyroidism in Pregnancy

In pregnant women, the most common causes of hypothyroidism are chronic autoimmune thyroiditis (Hashimoto's disease), subacute thyroiditis, history of radioactive iodine therapy, previous thyroidectomy, and iodine deficiency, with Hashimoto's disease being the most frequent cause in industrialized nations and iodine deficiency being the most common cause worldwide. 1

Etiology of Hypothyroidism in Pregnancy

Primary Causes

1. Autoimmune Thyroid Disease

   • Hashimoto's thyroiditis (chronic autoimmune thyroiditis) - most common cause in developed countries 1, 2
   • Characterized by presence of anti-thyroid peroxidase (anti-TPO) antibodies

2. Iodine-Related Causes

   • Iodine deficiency - most common cause worldwide 1
   • Excessive iodine intake can also trigger hypothyroidism in susceptible individuals

3. Iatrogenic Causes

   • Previous radioactive iodine therapy for hyperthyroidism 1
   • Prior thyroidectomy (partial or complete) 1

4. Other Causes

   • Subacute thyroiditis (post-viral inflammation of thyroid) 1
   • Hypothalamic dysfunction (rare) 1
   • Medications that interfere with thyroid function

Risk Factors for Developing Hypothyroidism During Pregnancy

• Personal or family history of thyroid disease 2
• Type 1 diabetes mellitus 2
• History of other autoimmune disorders 2
• Previous postpartum thyroiditis (70% recurrence risk in subsequent pregnancies) 2
• Living in areas with iodine deficiency

Clinical Presentation

Hypothyroidism in pregnancy may present with:

• Fatigue
• Muscle cramps
• Constipation
• Cold intolerance
• Hair loss
• Weight gain
• Intellectual slowness
• Insomnia 1

Advanced hypothyroidism can progress to myxedema, though this presentation is unusual during pregnancy 1.

Diagnostic Considerations

• TSH testing is the recommended initial screening test 1
• Both TSH and Free T4 (FT4) or Free T4 Index (FTI) should be measured in pregnant women with suspected hypothyroidism 1
• Anti-TPO antibodies help confirm autoimmune etiology 2
• Pregnancy alters thyroid function test results, requiring trimester-specific reference ranges 3

Clinical Implications

Untreated maternal hypothyroidism is associated with:

• Increased risk of preeclampsia 1
• Low birth weight in neonates 1
• Congenital cretinism (with iodine deficiency) 1
• Adverse obstetric outcomes 4
• Negative effects on fetal neurocognitive development 5

Management Considerations

• Women with pre-existing hypothyroidism typically need a 30-60% increase in levothyroxine dosage early in pregnancy 3
• The increased requirement varies based on the etiology of hypothyroidism 6
• TSH goal during pregnancy is <2.5 mIU/L 7
• Dosage requirements generally return to pre-pregnancy levels postpartum 5

Monitoring During Pregnancy

• Regular monitoring of thyroid function (TSH and FT4/FTI) every 4-6 weeks
• Dose adjustments as needed to maintain optimal thyroid function
• Particular attention during the first trimester when fetal development is most vulnerable to thyroid hormone deficiency

Understanding the cause of hypothyroidism in a pregnant woman is crucial for appropriate management and monitoring throughout pregnancy to ensure optimal maternal and fetal outcomes.