Determination of Thyroid Dysfunction and Reference Intervals during the third trimester in Port Harcourt, Nigeria

Background: Prevalence of thyroid disorders in pregnancy varies with reference intervals of thyroid function tests among various populations globally, considering differences in population-specific characteristics and geographical locations. Objective: To determine the prevalence of thyroid disorders in pregnant women in the third trimester in an iodine-sufficient city using internationally-recommended, assay-specific and laboratory-derived reference limits and to determine third trimester-specific reference intervals for thyroid function tests. Subjects and Methods: Serum thyrotropin (TSH), free thyroxine (FT4) and free triiodothyronine (FT3) were analysed in 178 pregnant women. Thyroid disorders were defined according to three criteria: the American Thyroid Association (ATA) third trimester-reference ranges, assay-specific non-pregnant reference intervals and laboratory-derived third trimester-reference intervals for thyroid function tests. Results: Using the ATA criteria, overall prevalence of thyroid disorders was 18.0%: subclinical hypothyroidism (12.4%), overt hypothyroidism (3.4%), overt hyperthyroidism (1.1%) and isolated hypothyroxinemia (1.1%). Using the assay-specific reference intervals, overall prevalence of thyroid disorders was 11.7%: subclinical hypothyroidism (7.3%), overt hypothyroidism (2.2%), overt hyperthyroidism (1.1%) and isolated hypothyroxinemia (1.1%). Using the laboratory-derived reference intervals, overall prevalence of thyroid disorders was 13.4%: subclinical hypothyroidism (8.4%), overt hypothyroidism (2.8%), overt hyperthyroidism (1.1%) and isolated hypothyroxinemia (1.1%). Laboratory-derived reference intervals for TSH, FT4 and FT3 were 0.40 – 3.70 mIU/L, 8.89 – 18.85 pmol/L and 2.88 – 6.05 pmol/L respectively. Conclusion: Laboratory-derived lower and upper reference intervals for TSH, FT4 and FT3 were observed to be lower than the assay-specific reference intervals and higher than the ATA third trimester TSH reference intervals.