Appy · 1 min
Thyroid, fertility, and the 'normal but not optimal' debate
1 section · 1 min read
What does 'normal TSH' mean for fertility, and why do doctors disagree?
TSH stands for thyroid-stimulating hormone. It is the most common thyroid blood test. When your TSH is high, your thyroid is underactive (hypothyroid). When it is low, your thyroid is overactive. The 'normal range' on most lab reports is roughly 0.4 to 4.0 milliunits per litre. Inside fertility clinics, you may hear a different number: 2.5.
The grey zone
Should TSH be below 2.5 if I am trying to conceive?
UK lab reference ranges treat TSH up to around 4.0 as normal. The original push for a 2.5 cutoff in fertility came from observational studies and expert opinion in the 2000s, not large randomised trials.
Where it gets more nuanced
What we honestly do not know
We do not know whether treating TSH between 2.5 and 4.0 in women with no other thyroid problem improves live birth rate. We do not have good data on whether South Asian women with PMOS and borderline TSH benefit differently.
Bottom line
If your TSH is between 2.5 and 4.0 and you are trying to conceive, the question is reasonable to raise but the evidence is mixed. Antibody status and a recent thyroid panel often matter more than the TSH number alone. A useful conversation with your doctor includes both numbers, your family history, and your fertility plan.
References
- [1] 27023449Plowden TC et al. Subclinical hypothyroidism and thyroid autoimmunity are not associated with fecundity, pregnancy loss, or live birth. J Clin Endocrinol Metab 2016;101(6):2358-2365.
For your doctor
Patient preparing for conception / fertility assessment. Requests TSH, free T4, and TPO antibodies. Discussion of subclinical hypothyroidism management and pre-conception target.
I would like a thyroid blood test, please. I am trying to conceive. Could we check TSH, free T4, and TPO antibodies, and discuss what 'normal' means for someone in my situation?
How did this land with you?
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References
- [1] 27023449Plowden TC et al. Subclinical hypothyroidism and thyroid autoimmunity are not associated with fecundity, pregnancy loss, or live birth. J Clin Endocrinol Metab 2016;101(6):2358-2365.
- [2] asrm-recurrent-pregnancy-loss-2020ASRM Practice Committee. Evaluation and treatment of recurrent pregnancy loss: a committee opinion. Fertil Steril 2020;113(3):533-535.
Reviewed by clinicians
Authored and reviewed by clinicians from the founding team. Information only, not personalised medical advice.