High Prevalence of Thyroid Disorders in Pregnant Women in a Mildly Iodine-deficient Country: A Population-Based Study

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 μg/g) had a significantly lower median Tg concentration compared to moderately iodine deficient women (UIC/Cr ≤ 49 μg/g), 19 μg/L and 25 μg/L, respectively. CONCLUSIONS: The prevalence of thyroid disorders was high, affecting one in six pregnant women in Belgium. Therefore, the iodine status in women needs to be improved and screening for thyroid disease should be performed early in pregnancy. In addition, our data suggest that a median Tg of <20 μg/L may indicate iodine sufficiency in pregnant women.