Thyroid Disorders, Fertility and IVF

Thyroid problems affect fertility mainly by disrupting ovulation, and they can also affect early pregnancy. Both an underactive and an overactive thyroid can cause irregular or absent cycles. The practical point is that thyroid disorders are common, often produce no obvious symptoms, are picked up by a simple blood test, and are treatable, which puts them among the most worthwhile things to rule out early. Thyroid function is usually checked as part of a standard fertility assessment for that reason.

How does the thyroid affect fertility?

The thyroid produces hormones that set the pace at which cells work. Reproductive function is one of the systems sensitive to them, so when levels are too high or too low the hormonal signalling that drives the menstrual cycle is disturbed.

In practice this shows up as:

• Irregular or absent periods, which usually means ovulation is not happening predictably.
• Cycles that appear normal but without ovulation.
• Effects on early pregnancy, with thyroid disorders associated with a raised risk of miscarriage.

Underactive and overactive thyroid

An underactive thyroid (hypothyroidism) is the more commonly discussed in fertility, and it can cause irregular cycles, absent ovulation and raised prolactin. An overactive thyroid (hyperthyroidism) can equally disturb cycles. Both matter, and both are treatable.

There is also a subtler category, sometimes called subclinical hypothyroidism, where thyroid hormone levels are within range but the signalling hormone TSH is raised. Whether and when to treat this in the context of fertility is genuinely debated among specialists, and thresholds differ. It is a question to put to your own doctor rather than one with a single answer.

Thyroid antibodies

Some people have antibodies against their own thyroid, indicating autoimmune thyroid disease, even while their thyroid hormone levels remain normal. This is associated with a raised risk of miscarriage. Whether it should change management is another area where practice varies, so ask what your own results mean.

What does this mean for IVF?

Thyroid function is normally checked before starting treatment, and where it is abnormal it is usually corrected first, because stimulation and pregnancy both place additional demand on the thyroid. Requirements also change during pregnancy, so someone already on treatment may need their dose reviewed.

If you are on thyroid medication and starting IVF, tell your fertility team and make sure someone is monitoring your levels through treatment and into pregnancy rather than assuming a stable dose will remain right.

How is it tested?

A blood test, usually measuring TSH with thyroid hormone levels, and sometimes thyroid antibodies. It is inexpensive and routinely included in fertility assessment. Our article on fertility blood tests covers what else is measured.

Symptoms such as fatigue, weight change, feeling cold or hot, and hair or skin changes can point to a thyroid problem, but many people have none, which is exactly why it is tested rather than waited for.

We at Millennium IVF Clinic assess thyroid function as part of fertility testing. You can arrange a consultation with our team.

Frequently asked questions

Can a thyroid problem be the only reason I am not conceiving?

It can be a significant contributing factor, particularly where cycles are irregular. It is one of the more treatable causes, which is why it is checked early.

If my thyroid is treated, will my fertility return?

Correcting thyroid function often restores regular cycles. Whether that resolves the whole picture depends on what else is going on, which is why both partners are assessed.

Should I take thyroid medication if my levels are borderline?

This is genuinely debated and depends on your specific results, your history and whether you are trying to conceive. Discuss it with your doctor rather than acting on a general rule.

Do I need my thyroid rechecked during pregnancy?

Thyroid requirements change in pregnancy, so anyone on treatment should have levels monitored. Establish who is responsible for that before you need it.