Knowledge Centre
Hormones & blood testsDraft: awaiting clinical review
Thyroid function
Both underactive and overactive thyroid can affect cycles, fertility and pregnancy.
Overview
Thyroid function is usually checked with TSH (thyroid-stimulating hormone), sometimes with free T4. An underactive thyroid typically raises TSH.
Thyroid conditions are common in women and are straightforward to test for.
What is well established
- Untreated overt hypothyroidism or hyperthyroidism can disrupt cycles and is associated with pregnancy complications; treatment is effective.
- People already taking levothyroxine often need their dose reviewed when trying to conceive and in early pregnancy.
Uncertain or emerging evidence
- Whether mild ('subclinical') hypothyroidism should be treated before pregnancy is debated, and target thresholds differ between guidelines.
- In people with normal thyroid function but positive thyroid antibodies, a large UK trial (TABLET) found that levothyroxine did not improve live birth rates.
This guide is general information and does not replace advice about your own circumstances. Clinical review pending.