Determination of Thyroid Dysfunction and Reference Intervals during the Third Trimester of Pregnancy 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, assayspecific and laboratory-derived reference limits and to determine third trimesterspecific 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 h y p o t h y r o i d i s m ( 3 . 4 % ) , o v e r t hyperthyroidism (1.1%) and isolated hypothyroxinemia (1.1%). Using the assayspecific 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 h y p o t h y r o i d i s m ( 2 . 8 % ) , o v e r t hyperthyroidism (1.1%) and isolated hypothyroxinemia (1.1%). Laboratoryderived 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 assayspecific reference intervals and higher than the ATA third trimester TSH reference intervals.