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Key learning points
Pregnant women do not need routine thyroid function testing for subclinical hypothyroidism (SCH). If thyroid function is tested, do not diagnose SCH if thyroid-stimulating hormone is within the normal reference range for pregnancy or <4.0mU/L.
There is no consistent evidence that SCH causes harm in pregnancy or that treatment with thyroxine is beneficial.
Any treatment for SCH in pregnancy is currently based on low-quality evidence. A low starting dose of thyroxine could be used (50 µg) and thyroid function tests reviewed after 4–6 weeks to ensure no harm from iatrogenic thyrotoxicosis.
More data are needed to inform the management of women with SCH who are positive for thyroid antibodies
Subclinical hypothyroidism (SCH) is defined as an elevated thyroid-stimulating hormone (TSH) concentration in combination with normal concentration of free thyroxine (T4). Systematic review and meta-analysis shows that thyroxine replacement in non-pregnant adults with SCH confers no benefit in either thyroid-related symptoms or quality of life.
In pregnancy, the definition of SCH requires gestation-specific reference intervals for TSH and T4, or a TSH between 4.0 and 10.0mU/L. SCH is estimated to affect 3%–5% of pregnant women. While overt hypothyroidism is recognised to be detrimental to pregnancy and treatment beneficial to pregnancy outcome and childhood development, it remains uncertain whether risk and the treatment benefit extend into the subclinical spectrum. This article considers the impact of pregnancy on the thyroid gland and thyroid function tests. Reference intervals for thyroid function are outlined including the new upper limit for TSH defined by the American Thyroid Association in 2017. Pregnancy outcomes in SCH are discussed and the evidence for thyroxine replacement is detailed.
Impact of pregnancy on the thyroid gland
The endocrine and haemodynamic changes of pregnancy mean that it is often viewed as a physiological ‘stress test’ for different organ systems. This is apparent in the thyroid where the net effect …
Competing interests None declared.
Provenance and peer review Commissioned; externally peer reviewed.