TSH Target for Infertility Treatment
For women undergoing infertility treatment, maintain TSH below 2.5 mIU/L before conception attempts, with treatment initiated when TSH exceeds this threshold. 1, 2
Evidence-Based TSH Threshold
The preconception TSH target of <2.5 mIU/L is recommended for all women seeking infertility care, whether they have clinical hypothyroidism, subclinical hypothyroidism, or are undergoing assisted reproductive technology (ART). 1, 2
Women with TSH levels >4.0 mIU/L experience impaired fertilization rates, reduced embryo quality, and lower live birth rates, which improve with levothyroxine therapy. 2
In women already on levothyroxine replacement, TSH should be maintained <2.5 mIU/L before starting ART, as ovarian stimulation places additional strain on the thyroid gland and can lead to permanent hypothyroidism in those with thyroid autoimmunity. 2
Clinical Evidence Supporting the 2.5 mIU/L Target
Women with unexplained infertility have significantly higher TSH levels (median 1.95 mIU/L) compared to controls with only severe male factor infertility (median 1.66 mIU/L), and nearly twice as many women with unexplained infertility (26.9%) have TSH ≥2.5 mIU/L compared to controls (13.5%). 3
Thyroxine therapy in hypothyroid infertile women (both clinical and subclinical) significantly reduces the time to pregnancy from 5.2 ± 1.8 years to 0.5 ± 0.8 years, with 54% achieving conception during treatment. 1
The majority of conceptions (52.6%) occur between 6-12 months of thyroxine therapy once TSH is optimized to <2.5 mIU/L. 1
Treatment Algorithm
Screen all women presenting for infertility evaluation with TSH, free T4, and thyroid antibodies. 2
If TSH >2.5 mIU/L: Initiate levothyroxine therapy and recheck thyroid function every 6 weeks until TSH <2.5 mIU/L is achieved. 1
If TSH 2.5-4.0 mIU/L with thyroid autoimmunity: Consider levothyroxine on a case-by-case basis, particularly if undergoing ART. 2
If TSH >4.0 mIU/L: Levothyroxine is mandatory, as this level impairs fertility outcomes even without clinical symptoms. 2
Important Caveats for High-Normal TSH (2.5-5.0 mIU/L)
The evidence for treating TSH levels between 2.5-5.0 mIU/L in women with unexplained infertility shows conflicting results—one study found higher conception rates but paradoxically lower live birth rates (63% vs 84%) with levothyroxine treatment, though this was limited by small sample size. 4
Women undergoing IUI with TSH in the highest quartile (2.35-4.5 mIU/L) do not have lower live birth rates compared to those with lower TSH levels when not treated with levothyroxine, suggesting that treatment may not be universally beneficial in this range for IUI specifically. 5
Despite these conflicting findings, the preponderance of evidence and guideline recommendations support treating TSH >2.5 mIU/L before conception attempts, particularly for women undergoing ART where ovarian stimulation increases thyroid demands. 1, 2
Monitoring During Treatment
Recheck TSH every 6 weeks during dose titration until the target of <2.5 mIU/L is consistently achieved. 1
Once optimal TSH is reached, monitor thyroid function throughout fertility treatment, especially during ovarian stimulation protocols which can precipitate hypothyroidism. 2
Serial thyroid function testing is essential as thyroid demands increase during early pregnancy, requiring dose adjustments in many women. 2