Context: Many countries in Europe remain mildly iodine deficient but relatively few country-level data exist on mild iodine deficiency (MID) and its impact on thyroid function in pregnant women. Objective: To determine the prevalence of thyroid disorders in pregnant women in Belgium and to assess the association between iodine status and serum thyroglobulin (Tg). Design and Setting: We conducted a national survey of pregnant women in 55 obstetric clinics. Urinary iodine concentration corrected for creatinine (UIC/Cr) and thyroid function were measured. Results: The frequency of elevated serum TSH was 7.2%, indicating either subclinical hypothyroidism (6.8%) or overt hypothyroidism (0.4%). Among those women, 13.8% were thyroid peroxidase antibodies (TPO-Ab) positive. The frequency of low serum TSH was 4.1%, indicating either subclinical hyperthyroidism (3.6%) or overt hyperthyroidism (0.5%). In the entire population, the frequency of positive TPO-Ab and/or Tg antibodies positive women was 4%. Globally, the prevalence of thyroid disorders (abnormally high or low TSH) or thyroid autoimmunity features was 15.3% and 18.6% in first-trimester pregnant women. Women with an adequate iodine status (UIC/Cr = 150-249 mug/g) had a significantly lower median Tg concentration compared to moderately iodine deficient women (UIC/Cr