Your Cycle · 1 min
Thyroid, fertility, and the 'normal but not optimal' debate
Top things to know
TSH is the most common thyroid blood test; most labs call 0.4 to 4.0 normal.
Fertility clinics often aim for TSH below 2.5, but doctors disagree about this cutoff.
A large study found TSH between 2.5 and 10 was not clearly linked to reduced fertility.
Data on South Asian women is limited, so your own numbers are worth discussing with your doctor.
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.
If you ever decide to raise it
Putting it into words
No words guarantee how an appointment goes. Precise words just make sure what you live with gets described, not rounded down.
Keep it, edit it, or just remember it. Stays on your phone.
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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.
From the team behind Her Holistic Health, founded by Dr Divpreet Sacha, NHS-trained GP.