TSH
What is TSH?
TSH stands for thyroid-stimulating hormone. It's made by the pituitary gland (a small gland at the base of the brain) and its job is to tell the thyroid gland in your neck how much thyroid hormone to produce. Measuring TSH is the standard blood test to check thyroid function.
A high TSH usually signals an underactive thyroid (hypothyroidism), a low TSH usually signals an overactive thyroid (hyperthyroidism).
Why might it matter for fertility?
Thyroid hormone can influence almost every tissue in the body, including the ovaries, testes, and the hormonal signalling that drives the menstrual cycle. When thyroid function is altered, there may be related:
Ovulation problems. Hypothyroidism is linked to irregular or absent periods, anovulatory cycles, and elevated prolactin, which itself can suppress ovulation.
Egg and embryo quality. Thyroid hormone receptors are present on egg cells and early embryos, so suboptimal levels can affect development.
Implantation and miscarriage risk. Untreated or borderline hypothyroidism is associated with higher rates of early miscarriage and pregnancy complications.
Sperm parameters. In men, thyroid dysfunction can reduce sperm count, motility, and morphology.
Autoimmune thyroid disease. Anti-thyroid antibodies (especially anti-TPO) can be associated with fertility issues and pregnancy loss even when TSH looks normal, which is why LCRH sometimes test for them too.
What does the latest research tell us about TSH testing?
New guidelines provide clear evidence that routine thyroid testing is only necessary in those with a personal or family history of thyroid disease or relevant clinical signs.
Limited evidence of impact on fertility: The data do not support that mild elevations in TSH adversely affect the chances of conception or pregnancy outcomes in most infertile individuals.
Reducing unnecessary interventions: Treating mild TSH elevations in patients without symptoms has not been shown to improve fertility outcomes and may lead to unnecessary treatment or stress.
LCRH will adopt a more targeted approach to ensure that testing is performed only when there are clinical indications, such as a personal or family history of thyroid disease, symptoms of thyroid dysfunction (e.g., fatigue, weight changes, or sensitivity to cold), or other risk factors, such as autoimmune conditions.
What are the key TSH levels in Fertility (mIU/L)?
The American Society for Reproductive Medicine (ASRM) no longer recommends a strict, one-size-fits-all preconception target.
TSH < 2.5 mIU/L: The universally accepted optimal range. Recommended primarily for women with previously diagnosed thyroid disease or those testing positive for thyroid autoantibodies (anti-TPO).
TSH 2.5 to 4.0 mIU/L: Considered normal for most women attempting pregnancy. Data shows this range is not associated with an increased risk of miscarriage.
TSH > 4.0 mIU/L : This is typically classified as subclinical hypothyroidism.
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