Thyroid function testing prior to fertility treatment: will we ever agree?

Thyroid disorders are among the most prevalent of medical conditions, especially in women, with the prevalence of both hypo- and hyper-thyroidism being 10-fold more common in women than in men. Thyroid hormones play a key role in subfertility associated with ovulatory dysfunction and miscarriages. While overt thyroid disorders are more often clinically diagnosed, subclinical hypothyroidism (SCH) poses a particular challenge as these patients are usually asymptomatic while their risk for adverse outcomes is higher than in the general population. SCH is biochemically diagnosed when peripheral thyroid hormone levels are normal, but thyroid-stimulating hormone (TSH) levels are elevated; the condition may also be associated with thyroid autoimmunity (TAI). SCH and TAI may remain latent, asymptomatic or undiagnosed, but SCH can progress to overt hypothyroidism in women with TAI during ovarian stimulation and pregnancy. The lack of consensus on the upper limit of normal for TSH has led to variations in the diagnosis of SCH and therefore variations in thresholds for potential treatment. Consequently, there are variations in recommendations for routine testing in patients seeking fertility treatment. Guidance from the British Thyroid Association, American Thyroid Association (ATA), European Thyroid Association (ETA), and Royal College of Obstetrics and Gynaecology recommends screening using TSH with or without antibody testing while those from the National Institute for Health and Care Excellence and the latest from the American Society of Reproductive Medicine do not recommend routine testing. This paper outlines the discourse and varied recommendations related to routine thyroid function testing, with specific reference to the subfertile population of women, and highlights the discord in opinions that currently exist in this arena. An update of the ATA guideline, which will include women with subfertility and will be endorsed by multiple international bodies (including ESHRE and ETA), is eagerly anticipated.