Limited Utility of Free Triiodothyronine Testing.

Area of Science:

• Endocrinology
• Clinical Chemistry
• Thyroidology

Background:

• Free triiodothyronine (fT3) testing is indicated when thyroid stimulating hormone (TSH) is suppressed and free thyroxine (fT4) is normal or decreased, a condition known as T3 thyrotoxicosis.
• Established guidelines for fT3 reflex testing are lacking, necessitating an evaluation of its clinical utility.

Purpose of the Study:

• To determine the clinical utility of fT3 testing.
• To identify an optimal TSH cutoff value for fT3 reflex testing in patients with normal or decreased fT4 levels.

Main Methods:

• Retrospective analysis of TSH, fT4, and fT3 laboratory results from January 2016 to October 2021.
• Evaluation of the frequency of biochemical T3 thyrotoxicosis at various TSH cutoffs.
• Comparison of T3 thyrotoxicosis prevalence in outpatient versus inpatient settings.

Main Results:

• Biochemical T3 thyrotoxicosis was identified in 1.6% of 4366 TSH-fT4-fT3 results.
• The detection rate of T3 thyrotoxicosis increased significantly with lower TSH cutoffs (<0.01 μIU/mL showed a 27.6% detection rate).
• Newly diagnosed hyperthyroidism was exclusively observed in patients with TSH <0.01 μIU/mL. T3 thyrotoxicosis was more frequent in outpatients (34%) than inpatients (14%) when TSH < 0.01 μIU/mL.

Conclusions:

• T3 thyrotoxicosis is a rare diagnosis, and fT3 measurement has limited utility in most patients.
• Implementing an fT3 reflex test for patients with TSH <0.01 μIU/mL and normal/low fT4 can enhance clinical utility.
• This approach may reduce unnecessary fT3 testing, particularly in the outpatient setting.