Trimester-specific thyroid hormone reference data in Sri Lankan women

Regional differences in thyroid hormones are noted, especially during pregnancy.

Establish reference values for thyroid function tests for Sri Lankan pregnant women and to determine their comparability with regional data; and determine the prevalence of 2. Thyroid Peroxidase (TPO) antibody positivity and 3. Iodine deficiency among pregnant women with uncomplicated clinical history.

A cross-sectional study conducted in antenatal clinics of a tertiary care maternity center recruited a minimum of 56 women in each trimester in a multistep approach to derive an "ideal-reference population"; Participants with clinically manifested thyroid disease, followed by subjects with sonographically abnormal thyroids and finally those at high risk for thyroid disease as shown by positive TPO levels and urine iodine deficiency were excluded in sequence. Thyroid hormones were measured by chemiluminescence in the ideal reference population. Reference ranges were derived using median and 5th and 95th centiles.

Final sample included 369 women. TSH reference ranges of the first (n=64), second (n=188) and third (n=117) trimesters were 0.014-2.77mIU/L, 0.31-3.2 mIU/L and 0.34-3.4 mIU/L, respectively. TPO antibody level showed a weak but significant correlation with TSH (r=0.10,p 0.021) in the final sample. No significant association was found between urine iodine and thyroid function tests.

TSH reference ranges observed in this study are concordant with the Caucasian reference values more than the regional values. Discrepancies in study methodology, defining and selection of reference population and methods employed in measuring thyroid hormones in different studies may have accounted for these differences.