The aim was to assess the proportion of women requiring a referral to an endocrinologist when carrying out routine screening for maternal thyroid disease simultaneously with screening for Down syndrome. Between November 2009 and September 2015, women having a first trimester combined test were screened; those symptomatic or being investigated for thyroid problems were excluded. In the first half of the period the blood sample was tested for anti-thyroid peroxidase (TPO) antibodies, serum thyroid stimulating hormone (TSH), and free thyroxine (fT4); thereafter fT4 was not used. Women with at least one analyte outside the range (TPO more than 5.6 kU/L, TSH 0.35-4.94 mU/L, fT4 9.0-19.1 pmL/L) were referred to an endocrinologist for counseling. Of 10,052 women screened, 1190 (11.8%) had TSH, fT4, or antibody levels outside the range; 13.0% when fT4 was used and 10.4% thereafter. Most of these positive results were due to a single abnormal marker. There was a much higher positive rate in multiple pregnancies (27.3% compared with 11.5% in singletons; P < 0.0001). There was an association between positivity and maternal age (P < 0.0001), a higher rate in the small ethnic group of Asians (P < 0.0005) and a lower rate among smokers (P < 0.02) and following assisted reproduction (P < 0.05). Routine antibody and hormone testing at the same time as the combined test generated a large number of referrals but this did not overload the services.