In a woman with Hashimoto’s thyroiditis who is planning pregnancy or is already pregnant, how should thyroid function be optimized—including target TSH levels, levothyroxine dose adjustments, monitoring frequency, iodine intake, and postpartum management?

Optimizing Thyroid Function in Hashimoto's Thyroiditis During Pregnancy

Preconception Optimization

Women with Hashimoto's thyroiditis planning pregnancy should target a preconception TSH ≤1.5–1.7 mIU/L (not the standard 2.5 mIU/L) to prevent first-trimester thyroid insufficiency. 1

Preconception TSH Targets Based on Treatment Status

• For women already on levothyroxine (hypothyroid Hashimoto's): Target preconception TSH ≤1.24 mIU/L to ensure first-trimester TSH remains <2.5 mIU/L, or ≤1.74 mIU/L if targeting first-trimester TSH <4.0 mIU/L 1
• For euthyroid women with Hashimoto's (not on levothyroxine): Target preconception TSH ≤1.73 mIU/L to ensure first-trimester TSH <2.5 mIU/L, or ≤2.07 mIU/L if targeting <4.0 mIU/L 1
• Preconception TSH values exceeding these cutoffs increase the risk of first-trimester thyroid insufficiency by 15–17 fold 1

Preconception Dose Adjustment Strategy

• Increase levothyroxine dose preconceptionally in women with compensated hypothyroidism to achieve partially suppressive TSH levels (lower quartile of normal range) 2
• This preconception adjustment results in adequate maternal thyroid function up to the first post-conception evaluation, particularly in thyroidectomized women 2
• Women receiving substitutive (rather than partially suppressive) doses preconceptionally show 36% frequency of low-normal free T4 in early pregnancy 2

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Immediate Pregnancy Confirmation Actions

Upon pregnancy confirmation, immediately increase levothyroxine dose by 25–50% (typically 25–50 mcg/day) in women with pre-existing hypothyroidism. 3

Rationale for Immediate Dose Increase

• Levothyroxine requirements increase during early pregnancy in women with pre-existing hypothyroidism 3
• Untreated maternal hypothyroidism increases risk of spontaneous abortion, gestational hypertension, pre-eclampsia, stillbirth, premature delivery, and adverse fetal neurocognitive development 3
• Over 80% of pregnant women with Hashimoto's thyroiditis require increased levothyroxine during pregnancy 4, 5

Initial Dosing for New-Onset Hypothyroidism in Pregnancy

• TSH ≥10 mIU/L: Start levothyroxine 1.6 mcg/kg/day 3
• TSH <10 mIU/L: Start levothyroxine 1.0 mcg/kg/day 3

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TSH Targets During Pregnancy

Maintain TSH <2.5 mIU/L in the first trimester and <3.0 mIU/L in the second and third trimesters. 3, 5

Trimester-Specific Monitoring

• Check TSH and free T4 as soon as pregnancy is confirmed 3
• Monitor TSH every 4 weeks until stable dose achieved and TSH within trimester-specific range 3
• After stabilization, check TSH at minimum once per trimester 3
• Adjust levothyroxine by 12.5–25 mcg increments based on TSH results 3

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Patterns of Levothyroxine Requirements During Pregnancy

Two Distinct Patterns in Hashimoto's Thyroiditis

Pattern 1 (32% of patients): Single dose increase or no change throughout pregnancy (T1 = T2 = T3), with mean TSH 1.3 mIU/L during gestation 4

Pattern 2 (53% of patients): Multistep dose increases required throughout pregnancy (T1 < T2 < T3), with mean TSH 2.8 mIU/L during gestation 4

Predictors of Increased Levothyroxine Requirements

• Preconception TSH level: Higher preconception TSH independently predicts greater dose increases (β = 0.507, p = 0.008) 5
• Inferior thyroid artery peak systolic velocity (ITA-PSV): Lower ITA-PSV independently predicts greater dose increases (β = -0.362, p = 0.047), reflecting reduced residual thyroid function 5
• Women with Pattern 2 are 62 times more likely to require levothyroxine dose ≥20% above baseline at 3 months postpartum 4

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Postpartum Management

Immediately reduce levothyroxine to pre-pregnancy dose after delivery. 3

Postpartum Monitoring Protocol

• Check TSH 4–8 weeks postpartum 3
• Postpartum TSH levels typically return to preconception values 3
• However, >50% of women with Hashimoto's thyroiditis experience persistently increased levothyroxine requirements postpartum compared to pregestational doses 4
• This persistent increase reflects postpartum progression of autoimmune thyroid destruction and depends on preexisting thyroid functional reserve 4

Postpartum Dose Adjustment

• If TSH remains elevated 4–8 weeks postpartum, increase levothyroxine by 12.5–25 mcg 6
• Recheck TSH 6–8 weeks after any dose adjustment 6
• Once stable, monitor TSH every 6–12 months 6

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Iodine Intake Considerations

Ensure adequate but not excessive iodine intake during pregnancy and lactation.

• Pregnant and lactating women require 220–290 mcg iodine daily (vs. 150 mcg for non-pregnant adults) 7
• Avoid excessive iodine exposure (e.g., kelp supplements, radiographic contrast) as this may exacerbate thyroid dysfunction in Hashimoto's thyroiditis 6
• Standard prenatal vitamins typically contain 150 mcg iodine 7

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Critical Safety Considerations

Levothyroxine Monotherapy Only

Use levothyroxine monotherapy exclusively during pregnancy—never combination T4/T3 therapy. 7

• T3 supplementation provides inadequate fetal thyroid hormone delivery 7
• Levothyroxine crosses the placenta and is essential for fetal neurodevelopment, particularly in the first and second trimesters 7

Medication Timing and Absorption

• Take levothyroxine on an empty stomach, 30–60 minutes before breakfast 7
• Separate from iron, calcium supplements, or antacids by at least 4 hours 6

Adrenal Insufficiency Screening

• Before initiating levothyroxine in suspected central hypothyroidism, rule out concurrent adrenal insufficiency by checking morning cortisol and ACTH 6
• Starting thyroid hormone before corticosteroids can precipitate life-threatening adrenal crisis 6, 3

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Common Pitfalls to Avoid

Preconception Phase

• Pitfall: Targeting preconception TSH of 2.5 mIU/L (standard non-pregnant target)
• Solution: Target TSH ≤1.5–1.7 mIU/L preconceptionally in Hashimoto's thyroiditis to prevent first-trimester insufficiency 1

Early Pregnancy

• Pitfall: Waiting for TSH results before increasing levothyroxine dose

• Solution: Increase dose by 25–50% immediately upon pregnancy confirmation, then adjust based on TSH results 3

• Pitfall: Underestimating levothyroxine requirements in women with Pattern 2 response

• Solution: Monitor TSH every 4 weeks and be prepared for multistep dose increases throughout pregnancy 4, 5

Postpartum Period

• Pitfall: Assuming all women return to pre-pregnancy levothyroxine requirements

• Solution: Recognize that >50% of women with Hashimoto's thyroiditis require persistently higher postpartum doses due to progressive autoimmune destruction 4

• Pitfall: Delaying postpartum TSH check beyond 8 weeks

• Solution: Check TSH 4–8 weeks postpartum to detect persistent hypothyroidism early 3

Monitoring Frequency

• Pitfall: Checking TSH too infrequently (e.g., once per trimester only)
• Solution: Check TSH every 4 weeks until stable, particularly in women with Pattern 2 response requiring multistep dose increases 3, 4