What should Thyroid-Stimulating Hormone (TSH) levels be when trying to conceive?

TSH Target When Trying to Conceive

Women trying to conceive should maintain TSH levels below 2.5 mIU/L, with an optimal target of less than 1.2 mIU/L to minimize the need for dose adjustments during early pregnancy.

Evidence-Based TSH Targets for Preconception

Primary Recommendation: TSH <2.5 mIU/L

• The Endocrine Society recommends maintaining preconception TSH levels below 2.5 mIU/L in women with hypothyroidism who are planning pregnancy 1, 2
• This target aligns with the established goal for pregnant women and helps prevent adverse maternal and fetal outcomes including preeclampsia, low birth weight, and impaired neuropsychological development 2
• Women with TSH levels between 2.5-4.5 mIU/L who are planning pregnancy should be treated with levothyroxine to achieve TSH <2.5 mIU/L 1

Optimal Target: TSH <1.2 mIU/L

• Research demonstrates that women with preconception TSH <1.2 mIU/L require dose increases during pregnancy only 17.2% of the time, compared to 50% of women with TSH 1.2-2.4 mIU/L 3
• Targeting TSH <1.2 mIU/L before conception significantly reduces the likelihood of requiring levothyroxine dose adjustments during early pregnancy when fetal neurological development is most vulnerable 3
• This lower target provides a safety buffer, as levothyroxine requirements typically increase by 25-50% during early pregnancy 1

Clinical Rationale for Strict TSH Control

Pregnancy-Related Thyroid Hormone Demands

• Maternal thyroid hormone requirements increase substantially during early pregnancy, often before the first prenatal visit 3
• Inadequate treatment of hypothyroidism during pregnancy is associated with preeclampsia, low birth weight, and potential neurodevelopmental effects in offspring 1, 2
• Maternal hypothyroxinemia (low T4 with normal TSH) has been linked to alterations in fetal neuropsychological development and increased risk of fetal loss 2

Impact on Fertility Outcomes

• Women with unexplained infertility have significantly higher TSH levels (median 1.95 mIU/L) compared to controls (median 1.66 mIU/L), even within the normal range 4
• Nearly twice as many women with unexplained infertility (26.9%) had TSH ≥2.5 mIU/L compared to controls (13.5%) 4
• Thyroxine therapy targeting TSH <2.5 mIU/L enhances fertility in infertile women with both clinical and subclinical hypothyroidism, with 54% achieving pregnancy during treatment 5

Treatment Algorithm for Women Planning Pregnancy

Initial Assessment

• Measure TSH and free T4 in all women seeking fertility care or planning pregnancy 1, 5
• Check anti-TPO antibodies to identify autoimmune thyroid disease, which predicts higher progression risk to overt hypothyroidism (4.3% per year vs 2.6% in antibody-negative individuals) 1

Treatment Initiation Based on TSH Levels

• TSH >2.5 mIU/L: Initiate levothyroxine therapy immediately to achieve TSH <2.5 mIU/L, ideally <1.2 mIU/L 1, 3
• TSH 1.2-2.5 mIU/L: Consider levothyroxine therapy or dose adjustment to achieve TSH <1.2 mIU/L, particularly in women with positive anti-TPO antibodies or unexplained infertility 4, 3
• TSH <1.2 mIU/L: Continue current management and monitor 3

Monitoring During Preconception Period

• Recheck TSH and free T4 every 6-8 weeks while titrating levothyroxine dose to achieve target TSH <1.2 mIU/L 1
• Once target TSH is achieved, recheck every 6-12 months or if attempting conception 1

Important Considerations and Caveats

Assisted Reproductive Technology

• For women undergoing IVF with euploid embryo transfer, maintaining TSH ≤2.5 mIU/L is sufficient, with no evidence that further optimization within this range improves implantation or live birth rates 6
• However, the TSH <1.2 mIU/L target remains prudent for natural conception attempts to minimize early pregnancy dose adjustments 3

Levothyroxine Dosing Strategy

• For women <70 years without cardiac disease, start with full replacement dose of approximately 1.6 mcg/kg/day 1
• Adjust dose in 12.5-25 mcg increments based on TSH response 1
• Anticipate that levothyroxine requirements will increase by 25-50% during early pregnancy, requiring proactive dose adjustments 1

Critical Pitfalls to Avoid

• Do not delay treatment in women with TSH >2.5 mIU/L who are actively trying to conceive, as early pregnancy thyroid hormone demands begin immediately after conception 2, 3
• Avoid treating based on a single elevated TSH value—confirm with repeat testing after 3-6 weeks, as 30-60% of elevated TSH levels normalize spontaneously 1
• Never assume that TSH 2.5-4.5 mIU/L is acceptable for women planning pregnancy, as this range is associated with increased fertility complications and will likely require urgent dose increases during early pregnancy 4, 3