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):

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 your levothyroxine is increased immediately after your pregnancy is confirmed, usually by 25-50mcg daily. This is most easily achieved by doubling your current dose on two days of the week. You should then contact your GP and arrange to have a thyroid blood test .

See our pregnancy and thyroid disorders alert card