What is the optimal Thyroid-Stimulating Hormone (TSH) level for fertility in men?

Optimal TSH for Male Fertility

For men seeking fertility, maintain TSH levels below 2.5 mIU/L, as higher TSH values within the "normal" laboratory range are associated with impaired spermatogenesis and reduced fertility outcomes.

TSH Targets and Male Reproductive Function

The relationship between thyroid function and male fertility is bidirectional and clinically significant:

• TSH levels should be optimized to <2.5 mIU/L in men with fertility concerns, as this represents the true physiologic normal range rather than the broader laboratory reference intervals 1
• Women with unexplained infertility have significantly higher TSH levels (median 1.95 mIU/L) compared to controls with only male factor infertility (median 1.66 mIU/L), suggesting that TSH levels in the 2.0-2.5 mIU/L range may already reflect subclinical thyroid dysfunction affecting fertility 2
• Thyroid dysfunction disrupts the hypothalamic-pituitary-gonadal axis, leading to abnormal FSH and LH secretion that impairs spermatogenesis 3

Clinical Evidence Supporting Lower TSH Targets

The evidence strongly supports treating to lower TSH thresholds in the fertility population:

• Hyperthyroidism causes specific reproductive changes including higher rates of asthenozoospermia, oligozoospermia, and teratozoospermia, all of which are reversible with thyroid treatment 3
• Inadequate thyroid replacement elevates SHBG and impairs spermatogenesis through disruption of the hypothalamic-pituitary-gonadal axis 3
• Correction of thyroid dysfunction improves semen quality, making it essential to achieve euthyroid status before making definitive conclusions about fertility status 3

Practical Management Algorithm

Initial Assessment

• Measure TSH, free T4, and thyroid antibodies in all men presenting with infertility 3
• Check FSH, LH, testosterone, and prolactin to evaluate the complete hypothalamic-pituitary-gonadal axis 4, 3
• Avoid hormonal testing during acute illness or metabolic stress, as transient conditions artificially affect the axis 4, 3

Treatment Targets

• Target TSH to 0.4-2.5 mIU/L for optimal fertility outcomes 1
• If TSH is in the upper half of the reference range (>2.5 mIU/L), increase levothyroxine dose to bring TSH into the lower portion of the reference range 3
• Recheck thyroid function every 6 weeks until optimal levels are achieved 5

Monitoring and Follow-up

• Once TSH is optimized to <2.5 mIU/L, perform semen analysis after 2-3 months to allow time for spermatogenic cycle completion 3
• Maximum fertility improvement occurs between 6-12 months of optimized thyroid therapy 5
• Continue monitoring TSH every 3-6 months to maintain optimal levels 5

Critical Pitfalls to Avoid

• Do not accept TSH values of 2.5-4.5 mIU/L as "normal" in men seeking fertility—these levels are associated with impaired reproductive outcomes despite falling within standard laboratory reference ranges 1, 2
• Never prescribe testosterone therapy to men with thyroid dysfunction and fertility concerns, as it will completely suppress spermatogenesis through negative feedback on the hypothalamus and pituitary 4, 3
• Do not perform semen analysis during periods of uncontrolled thyroid dysfunction, as results will be misleading and not representative of true fertility potential 3
• Avoid making definitive fertility diagnoses until thyroid function has been optimized for at least 3-6 months 3, 5

Integration with Other Fertility Parameters

When evaluating men with both thyroid dysfunction and elevated FSH:

• Address thyroid dysfunction first before attributing fertility problems solely to elevated FSH, as thyroid correction may normalize FSH levels 3
• Men with borderline FSH levels (9-12 IU/L) should undergo repeat hormonal testing after thyroid optimization, as FSH often normalizes to 7-9 IU/L once thyroid function is corrected 3
• High SHBG associated with thyroid dysfunction does not directly impair sperm production if LH remains normal, as spermatogenesis depends on intratesticular testosterone driven by LH stimulation 3

Evidence Quality Considerations

The recommendation for TSH <2.5 mIU/L is supported by:

• Cross-sectional data showing significantly higher TSH levels in women with unexplained infertility, with nearly twice as many having TSH ≥2.5 mIU/L compared to controls (26.9% vs 13.5%) 2
• Cohort data demonstrating that thyroxine therapy targeting TSH <2.5 mIU/L resulted in 54% conception rate in previously infertile hypothyroid women 5
• Physiologic data establishing that the true normal TSH range is 0.4-2.5 mIU/L based on populations without thyroid disease 1