SummaryObjectiveRecent studies report high rates of thyroid disorders in pregnant women. However, the need for universal thyroid screening remains controversial. Our aim was to estimate the prevalence of thyroid dysfunction (TD) during pregnancy and to analyse the association with maternal age.Design and methodsWe conducted a cross‐sectional study in a referral centre in collaboration with the primary care units from April 2010 to March 2011. The study included 2509 consecutive pregnant women resident in an iodine‐sufficient area, mean age 32 years (range 16–47) who were universally screened for TD in their first trimester (median gestation 8 weeks, range 4–13 weeks). Thyroid‐stimulating hormone (TSH) and free T4 (FT4) were analysed during the first antenatal visit. We applied first trimester‐specific population‐based TSH and FT4 reference ranges.ResultsWe identified 416 women with positive TD screening [16·6%, 95% confidence interval (95% CI) 15·1–18·0]. Of these, 47 had overt hypothyroidism (1·9%), 90 subclinical hypothyroidism (3·6%), 23 overt hyperthyroidism (0·9%), 20 subclinical hyperthyroidism (0·8%) and 236 had isolated hypothyroxinaemia (9·4%). Applying a logistic regression model, age ≥30 years was not associated with a higher risk of TD [odds ratio (OR) 0·85, 95% CI 0·67–1·08] or hypothyroidism (OR 0·72, 95% CI 0·50–1·06).ConclusionsTD affects one in six pregnant women in an iodine‐sufficient population. Maternal age ≥30 years do not increase the risk of TD.