The thyroid and fertility: what TSH changes, and what it doesn’t

Many patients arrive with a TSH level of 2.8 or 3.2 and one question: should it be treated? Recommendations have evolved in recent years, and practices vary from one clinic to another. Here is what is generally accepted, and what is still under discussion.

Why the thyroid matters

Thyroid hormones play a role in ovulation, implantation and early pregnancy development, particularly the development of the baby’s brain during the first trimester, a period when the foetus is entirely dependent on the mother’s hormones. Untreated hypothyroidism is associated with ovulation disorders, a higher rate of miscarriages and pregnancy complications.

Tests

• TSH: the basic test. Levels rise when the thyroid is underactive.
• Free T4: if the TSH is abnormal.
• Anti-TPO antibodies: indicative of autoimmune thyroiditis (Hashimoto’s), which is common in young women.

Clear-cut situations

• Overt hypothyroidism (high TSH, low T4): this must be treated with levothyroxine and stabilised before IVF.
• Hyperthyroidism (very low TSH, high T4): this must be managed by an endocrinologist before any pregnancy; certain treatments may need to be adjusted.
• TSH greater than approximately 4 mIU/L: most professional bodies recommend treatment prior to IVF.

The grey area: TSH between 2.5 and 4

For a long time, the target was a TSH level below 2.5 prior to pregnancy. More recent data are more nuanced. A TSH level between 2.5 and 4 in a woman without antibodies does not appear to reduce the chances of a birth after IVF. The American Society for Reproductive Medicine considers that treatment may be discussed, particularly in cases of positive antibodies or a history of miscarriages, but that it is not systematically necessary.

In practice, many clinics, in Greece as elsewhere, prefer a TSH level below 2.5 and prescribe a low dose of levothyroxine: the treatment is inexpensive and risk-free at these doses if monitored. This is not a mistake; it is a precaution.

Anti-TPO antibodies with normal TSH levels

These are associated with a slightly higher risk of miscarriage. However, two large randomised trials — TABLET in the UK (New England Journal of Medicine, 2019) and a Chinese IVF trial (BMJ, 2017) — have shown that giving levothyroxine to these women, when their TSH is normal, does not increase the birth rate. What matters, therefore, is monitoring: these women are at higher risk of their TSH levels rising during pregnancy.

During stimulation and pregnancy

Stimulation and pregnancy increase the body’s need for thyroid hormones. In a woman already on treatment, the dose often needs to be increased as soon as the test is positive — typically by around 25 to 30 per cent, at the doctor’s discretion. A follow-up TSH test is useful every four weeks during the first trimester.

What is of no use

Selenium, high-dose iodine or so-called ‘thyroid cures’ have shown no benefit for fertility and may upset the balance of a fragile thyroid. Iodine is included in pregnancy vitamins at normal doses; beyond that, it should not be taken without medical advice.

For your file

Have your TSH and anti-TPO levels tested during the initial assessment, and bring the results with you to your first consultation. If you are on treatment, note down the dose and the date of your last blood test. This simple piece of information helps to ensure you don’t lose a cycle. See also preparing in the three months before.

Frequently asked questions