Background: The interpretation of thyroid hormone function during pregnancy is difficult due to its physiologic changes. Differences in iodine status in previous studies led to different intervals; therefore the use of trimester-specific, method-specific and probably country-specific reference values is advocated. Objective: To establish trimester-specific reference interval for thyroid function tests in pregnant Filipino women. Methodology: Six hundred sixteen pregnant patients (5–40 weeks gestation) attending a tertiary center were recruited. Level of serum thyroid stimulating hormone (TSH) was measured using immunoradiometric assay while free thyroxine (FT4), free triiodothyronine (FT3) and thyroid peroxidase antibodies (TPOAb) were measured by radioimmunoassay method. Main outcome measures are trimester-specific reference interval based on 2.5th and 97.5th percentiles for TSH, FT4 and FT3 among TPOAb-negative pregnant patients. Results: The reference intervals for each trimester were as follows: TSH (0.05-4.24, 0.13-3.95, and 0.20-3.00 uIU/mL); FT4 (9.80-21.88, 9.10-18.95 and 9.16-18.64 pmol/L) and FT3 (2.40-6.20, 2.77-5.00 and 2.09-3.70 pmol/L). FT4 and FT3 are strongly and negatively correlated with age of gestation (p=<0.01and <0.01 respectively). No correlation is found with TSH and age of gestation (p=0.52). Conclusions Trimester-specific intervals among pregnant Filipino women are different from their non-pregnant counterparts and laboratory cutoffs. Thus, these reference values should be used in the country.
Thyroid Function Tests