What should be considered for a pregnant patient with hypothyroidism (underactive thyroid) to manage their condition effectively throughout the pregnancy?

Management of Hypothyroidism in Pregnancy

Immediately initiate levothyroxine upon diagnosis of hypothyroidism in pregnancy to prevent serious maternal and fetal complications including preeclampsia, low birth weight, placental abruption, and impaired neurodevelopment in the child. 1, 2

Initial Assessment and Diagnosis

When hypothyroidism is suspected in a pregnant patient, obtain both TSH and free T4 (or Free Thyroxine Index) for diagnosis 1, 2. TSH testing using monoclonal antibodies is the recommended initial screening test, but both values are essential for complete assessment 1.

High-risk women who require screening include: 3

• Previous treatment for hyperthyroidism
• History of high-dose neck irradiation
• Evidence of thyroid autoimmunity (positive TPO antibodies)
• Type 1 diabetes
• Suspected hypopituitarism
• History of amiodarone therapy

Treatment Initiation and Dosing

For Pre-existing Hypothyroidism

Women already on levothyroxine before pregnancy typically require a 30% or more dose increase by 4-6 weeks' gestation. 1 Measure serum TSH and free T4 as soon as pregnancy is confirmed 4. Increase the levothyroxine dose by 12.5 to 25 mcg per day when TSH is above the normal trimester-specific range 4.

For Newly Diagnosed Hypothyroidism in Pregnancy

• TSH ≥10 IU/L: Start levothyroxine at 1.6 mcg/kg/day 4
• TSH <10 IU/L: Start levothyroxine at 1.0 mcg/kg/day 4

The full replacement dose for most adults is 1.6 mcg/kg/day, though some patients require lower starting doses 4. Undertreatment poses far greater risks than overtreatment during pregnancy, with inadequate treatment associated with preeclampsia, preterm delivery, low birth weight, and impaired neurodevelopment 2.

Monitoring Schedule

Monitor TSH every 4 weeks after any dose change until stable, then continue monitoring throughout each trimester. 4 The goal is to maintain TSH within the trimester-specific reference range 4. For women on stable replacement therapy before pregnancy, check TSH at minimum during each trimester 4.

Adjust levothyroxine dosage by 12.5 to 25 mcg increments every 4 weeks as needed until the patient is euthyroid 4. Failure to monitor thyroid function regularly during pregnancy may result in inadequate treatment and serious complications. 2

Specific Risks of Untreated or Inadequately Treated Hypothyroidism

Maternal Complications: 1, 3, 5

• Pregnancy-induced hypertension (occurs in 22-44% of untreated cases)
• Preeclampsia
• Placental abruption
• Preterm delivery

Fetal and Neonatal Complications: 1, 3

• Low birth weight
• Fetal death and stillbirth
• Congenital cretinism (with iodine deficiency)
• Cognitive impairment and lower IQ in children
• Neuropsychologic defects
• Fetal distress in labor

First trimester hypothyroidism is particularly associated with cognitive impairment in children, making early detection and treatment critical. 1

Postpartum Management

Immediately reduce levothyroxine to pre-pregnancy levels after delivery. 4 Monitor serum TSH 4 to 8 weeks postpartum to ensure appropriate dosing 4. Thyroxine requirements typically decrease to preconception levels during the postpartum period 3.

Inform the newborn's physician about maternal thyroid disease due to risk of neonatal thyroid dysfunction. 2

Special Considerations

Thyroid Antibodies

Women with positive thyroid peroxidase (TPO) antibodies have double the risk of miscarriage and increased risk of postpartum thyroiditis 3. One study reported lower IQs in children of euthyroid women with positive TPO antibodies compared to TPO-antibody negative mothers 3.

Inadequate Response to Treatment

If TSH remains elevated despite apparently adequate levothyroxine dosing, evaluate for: 4, 6

• Poor compliance (most common cause)
• Malabsorption
• Drug interactions
• Combination of these factors

Dosages greater than 200 mcg/day are seldom required, and inadequate response to daily dosages greater than 300 mcg/day is rare 4.

Screening Controversy

Targeted screening of only high-risk women misses approximately 30-55% of hypothyroidism cases in pregnant women. 7 While most professional societies do not recommend universal screening and instead advocate a case-finding approach, some researchers recommend universal screening to facilitate early detection and treatment of overt hypothyroidism 7.