Thyroid dysfunction and autoimmunity are associated with adverse fertility and pregnancy outcomes. International bodies recommend routine thyroid function screening in women with history of subfertility or miscarriage. Knowledge about the frequency of, and risk factors for, thyroid disease is limited in the asymptomatic preconception population. A prospective multi-centre study of women with history of miscarriage or subfertility conducted at 49 hospitals across the United Kingdom between 2011 and 2016. Thyroid function tests and anti-thyroid peroxidase antibodies (TPOAb) were recorded in 19 213 and 19 237 women respectively. The overall prevalence of abnormal thyroid function was 4.8% (95% CI 4.55.1), with euthyroidism defined as thyroid stimulating hormone (TSH) 0.444.50 mIU/l and free thyroxine (fT4) 1021 pmol/l. Overt hypothyroidism (TSH >4.50 mIU/l and fT4 <10 pmol/l) was present in 0.2% (95% CI 0.10.3) and overt hyperthyroidism (TSH <0.44 mIU/l and fT4 >21 pmol/l) in 0.3% (95% CI 0.20.3). The prevalence of subclinical hypothyroidism (SCH) using an upper TSH concentration of 4.50 mIU/l was 2.4% (95%CI 2.12.6). Lowering the upper TSH limit to 2.50 mIU/l, a commonly adopted practice, resulted in a higher rate of SCH of 19.9% (95%CI 19.320.5). Multiple regression analyses found increased odds of SCH (TSH >4.50 mIU/l) with body-mass index (BMI) ≥35.0 kg/m2 (aOR 1.71 (1.132.57) P=0.01) and Asian ethnicity (aOR 1.76 (1.312.37) P<0.001), as well as increased odds of SCH (TSH ≥2.50 mIU/l) with subfertility (aOR 1.16 (1.041.29) P=0.008). TPOAb positivity was found in 9.5% (95%CI 9.1-9.9). BMI ≥35.0 kg/m2 and TSH concentrations ≥2.50 mIU/l were associated with greater odds of TPOAb positivity. Subclinical hypothyroidism and thyroid autoimmunity are common in women with history of miscarriage or subfertility; particularly in those with higher BMI and in Asian women. Applying a TSH cut-off of 2.5 mIU/l to define SCH results in a significant proportion of women potentially requiring levothyroxine treatment preconception and during pregnancy. The risks and benefits of this treatment strategy need to be evaluated further.