Do women with borderline (subclinical) hypothyroidism who are struggling to conceive need to have their thyroid levels optimised?

Untreated, or undertreated, hypothyroidism can make it harder to conceive and can result in poorer pregnancy outcomes. However, the evidence about the effect of borderline (subclinical) hypothyroidism on fertility is less clear-cut. If you have a diagnosis of subclinical hypothyroidism, and are not currently being treated for it with levothyroxine (L-T4), you should have a TSH measurement performed as soon as possible to see whether you need to start treatment.

The tables below provide a guide to the treatment of women with subclinical hypothyroidism not currently being treated with levothyroxine (L-T4). Please find our full guidance based on the Royal College of Obstetricians and Gynaecologists Green-top Guideline on the Management of Thyroid Disorders in Pregnancy in the link below.

	Thyroid peroxidase antibodies (TPO Ab) positive
TSH greater than reference range	Treatment with L-T4 recommended
TSH above 2.5 but less than reference range	Consider treating with L-T4
TSH less than 2.5*	Don’t treat

	TPO Ab negative
TSH greater than 10	Treatment with L-T4 recommended
TSH above reference range but less than 10	Consider treating with L-T4
TSH less than reference range (or less than 4.0)*	Don’t treat

  

*Some fertility clinics may treat people with a TSH greater than 2.5 mIU/L regardless of antibody status, but there is no real hard evidence for this.

If you are already receiving L-T4 treatment for overt hypothyroidism (or subclinical hypothyroidism), it is generally recommended that during conception your levothyroxine is adjusted to achieve a TSH of <2.5 mlU/L. As soon as you have a positive pregnancy test, increase your dose usually by 25-50mcg daily. This is most easily achieved by doubling your current dose on two days of the week. Do not wait to see your GP before doing this. Contact your GP and arrange to have a thyroid blood test.

Should all pregnant women have their thyroid function tested?

The RCOG guideline recommends that doctors do not need to check all pregnant women's thyroid levels or their thyroid peroxidase antibodies (TPOab) at their pregnancy booking appointment.

Find more guidance about managing thyroid conditions in conception and pregnancy