Urinary compound W in pregnant women is a potential marker for fetal thyroid function

OBJECTIVE: Previously we reported 3,3 ́-diiodothyronine sulfate–like material (compound W) in maternal serum, and studies suggest that compound W is derived from thyroid hormones of fetal origin. In this study we characterized gestational changes of urinary compound W concentrations to correlate with changes in serum concentrations. STUDY DESIGN: Urinary samples were collected from 94 women at various gestational ages ranging from 3 to 40 weeks. Urinary compound W was first identified biochemically. The concentrations of compound W (adjusted for creatinine levels) were assessed by a 3,3 ́-diiodothyronine sulfate radioimmunoassay in ethanol extracts of urine samples. RESULTS: Compound W increased to 88 ± 1.4 pmol (of 3,3 ́-diiodothyronine sulfate equivalent)/mmol creatinine in urinary samples obtained from 26 women in the first trimester of pregnancy compared with 40 ± 6.9 pmol/mmol creatinine in 10 nonpregnant women. Excretion of compound W increased further during the second and third trimesters: 171 ± 17 (n = 18) and 434 ± 26 (n = 50) respectively. In contrast, urinary 3,3 ́,5-triiodothyronine sulfate concentrations measured by radioimmunoassay were similar during pregnancy to values in nonpregnant women. CONCLUSIONS: Urinary compound W concentrations increase with the progression of normal pregnancy and correlate with the increase in serum levels. Random spot urine compound W concentrations, adjusted for creatinine levels, may be used in place of serum levels in conditions in which obtaining serum samples may be technically difficult, especially during population screening. (Am J Obstet Gynecol 1998;178:886-91.)