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CME Credit Offered
CME Credit Offered
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Abstract Award
Recording available 2/16-5/2
Recording available 2/16-5/2
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AIUM Credit
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Foundation Awardee
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Abstract Award
Abstract Award
Foundation Awardee
Foundation Awardee

Poster Session 2

Category: Medical/Surgical/Diseases/Complications

Poster Session 2

(445) Free thyroxine (FT4) assay in pregnancy routinely underestimates T4 compared to the adjusted Total T4

Wednesday, February 11, 2026
3:30 PM - 5:00 PM  
Objective:
Accurate thyroid function testing in pregnancy is critical to avoid under- and over-treatment. Free thyroxine (FT4) by immunoassay (FT4IA) is commonly used but limited in pregnancy due to marked binding protein changes, a FT4 fraction of < 0.05%, and lack of trimester-specific ranges. The ATA endorses total thyroxine (TT4) adjusted by 1.5x after 16 wks, but many clinicians are unaware.  We hypothesized FT4IA and pregnancy-adjusted TT4 disagree at a rate of 20% after 16 wks. We also compared TT4 to FT4 by equilibrium dialysis, tandem mass spectrometry (FT4ED), the gold standard with trimester-specific ranges.    

Study Design:
Retrospective study of pregnant individuals18-45 yrs between 1/2021-6/2025, >16 wks gestation, with paired FT4IA vs TT4 (Siemens ADVIA Centaur XP) samples, or TT4 vs FT4ED (ARUP) samples, within 7 days. Lab results were assigned to quintile groups (Below Normal range (NR); Lower NR; Mid NR; Upper NM; Above NR) as these ranges are targeted clinically. A binomial test assessed whether 20% of FT4IA and TT4 quintiles disagreed.  

Results:
FT4IA vs TT4
FT4IA and TT4 were measured in 78 pregnancies (mean age 32 yrs; Gravity/Parity (G/P) 2.6/1.1).  A binomial test of proportions found that the disagreement rate was 89.7% (95% CI: [80.8%-95.5%], p < 0.001). In of all disagreements (~90%), the FT4 quintile was lower than TT4 (Fig 1).  

TT4 vs FT4ED
TT4 and FT4ED were measured in 14 pregnancies (mean age 33 yrs; G/P 2.9/1.4) and were discordant in 28.6% (95% CI [8.4%-58.1%], p = 0.50). The TT4 quintile was higher than FT4ED in 3/4 of discordant cases (Fig 2).     

Conclusion:
We found only a ~10% agreement when FT4IA and adjusted TT4 quintiles were compared, with FT4IA consistently lower than TT4. In contrast, TT4 agreed with FT4ED in ~71% of quintiles. Clinical implications of using FT4IA include undertreatment with anti-thyroid meds for hyperthyroidism and overtreatment with LT4 for hypothyroidism. Our data supports an urgent need for trimester-specific FT4IA ranges and/or improved clinician education on use of TT4 or FT4ED for appropriate thyroid disease management in pregnancy.