What medications and dosages should be given to a pregnant patient in her 2nd trimester with gestational diabetes mellitus (GDM) and hypothyroidism?

Management of Gestational Diabetes Mellitus and Hypothyroidism in Second Trimester Pregnancy

Insulin is the first-line medication for treating gestational diabetes mellitus (GDM), while levothyroxine should be used to manage hypothyroidism, with dosage adjustments based on trimester-specific TSH levels. 1, 2

Gestational Diabetes Management

First-Line Approach: Lifestyle Modifications

• Begin with medical nutrition therapy:
  • Individualized nutrition plan providing adequate calories
  • Minimum 175g carbohydrates, 71g protein, and 28g fiber daily
  • Emphasize monounsaturated and polyunsaturated fats
  • Limit saturated fats and avoid trans fats
  • Focus on complex carbohydrates to minimize postprandial glucose excursions 1
• Recommend regular physical activity as tolerated

Blood Glucose Targets for GDM

Monitor blood glucose with the following targets:

• Fasting: <95 mg/dL
• 1-hour postprandial: <140 mg/dL
• 2-hour postprandial: <120 mg/dL 1

Pharmacological Management for GDM

• Initiate insulin therapy if lifestyle modifications fail to achieve target glucose levels 1
• Insulin is preferred because it does not cross the placenta to a measurable extent
• Do not use metformin or glyburide as first-line agents as they cross the placenta 1
  • Metformin has been associated with higher BMI and increased waist-to-height ratio in offspring 1, 3
  • Glyburide has been associated with increased neonatal hypoglycemia and macrosomia 1

Hypothyroidism Management

Levothyroxine Dosing

• For pre-existing hypothyroidism: Increase pre-pregnancy dose by 12.5-25 mcg/day 2
• For newly diagnosed hypothyroidism:
  • TSH ≥10 IU/L: Start at 1.6 mcg/kg/day
  • TSH <10 IU/L: Start at 1.0 mcg/kg/day 2

Monitoring Thyroid Function

• Measure serum TSH and free-T4 at minimum once each trimester 2
• Monitor TSH every 4 weeks after dosage changes until stable
• Maintain TSH within trimester-specific reference range 2
• Inadequate treatment of maternal hypothyroidism increases risk of pregnancy complications including preeclampsia, placental abruption, and impaired infant neurodevelopment 4

Monitoring Protocol

For GDM

• Check fasting and postprandial glucose levels daily
• Assess for need to initiate or adjust insulin therapy weekly
• Monitor for pregnancy complications including polyhydramnios, preeclampsia, and fetal macrosomia 5
• Consider fetal surveillance starting at 32 weeks if requiring medication 5

For Hypothyroidism

• Check TSH and free-T4 every 4 weeks until stable, then once per trimester 2
• Adjust levothyroxine dose to maintain TSH in trimester-specific reference range 2

Postpartum Considerations

• Insulin resistance typically resolves after delivery; discontinue insulin postpartum 5
• Reduce levothyroxine dosage to pre-pregnancy levels immediately after delivery 2
• Monitor TSH 4-8 weeks postpartum 2
• Screen for type 2 diabetes 4-12 weeks postpartum due to increased risk of developing overt diabetes 5, 6
• Encourage breastfeeding, which may reduce obesity in children 7

Important Caveats

• Do not use oral hypoglycemic agents during pregnancy as first-line therapy due to placental transfer and potential effects on the fetus 1
• Metformin should be discontinued by the end of the first trimester if it was being used for polycystic ovary syndrome 1
• Women with GDM have increased risk of developing type 2 diabetes later in life and require long-term follow-up 6, 7
• Women with both GDM and hypothyroidism may have higher risk of pregnancy complications and require more intensive monitoring 4