Second-trimester reference intervals for thyroid tests: the role of ethnicity

Thyroid function changes during pregnancy, complicating the diagnosis of thyroid disorders. Maternal thyroid dysfunction has been associated with a variety of adverse outcomes. We evaluated thyroid function test results by ethnicity and week of gestation during the 2nd trimester of pregnancy.

We collected 3064 blood specimens in serum tubes from Asians (13%), blacks (22%), Hispanics (23%), and whites (42%). We measured thyroid-stimulating hormone (TSH), total and free thyroxine (TT4 and FT4), total and free triiodothyronine (TT3 and FT3), thyroglobulin autoantibodies (TgAb), and thyroid peroxidase autoantibodies (TPOAb) by use of an ARCHITECT i2000(SR) (Abbott Diagnostics). The TSH reference interval was calculated for samples negative for both TgAb and TPOAb and reference intervals for TT4, FT4, TT3, and FT3 in antibody-negative samples with normal TSH.

Serum samples were positive for TgAb in 10.6%, 1.8%, 6.2%, 6.5%, and 5.9% of Asian, black, Hispanic, white, and combined groups, respectively. Samples were positive for TPOAb in 12.4%, 4.1%, 11.8%, 12.3%, and 10.4% of the same groups, respectively. The nonparametric reference intervals for all participants were 0.15-3.11 mIU/L (TSH), 9.3-15.2 pmol/L (0.72-1.18 ng/dL; FT4), 89.0-176.3 nmol/L (6.90-13.67 mug/dL; TT4), 3.82-5.96 pmol/L (2.48-3.87 pg/mL; FT3), and 1.82-3.68 nmol/L (118-239 ng/dL; TT3).

Blacks had lower prevalences of TgAb and TPOAb positivity and of increased serum TSH. The prevalence of TgAb and TPOAb positivity was highest in Asians. Whites had the highest prevalence of increased TSH. The lower and upper reference limits of TT3 were significantly lower for Asians. Reference intervals for women in the 2nd trimester were different from those of nonpregnant individuals.