USE OF THE FREE THYROXINE INDEX TO REFINE THE LOWER LIMIT OF A FREE THYROXINE IMMUNOASSAY FOR DETECTION OF SECONDARY...

To determine the utility of measuring free T₄ index (FT₄I) in patients with low free T₄ (FT₄) levels using immunoassay and normal thyroid-stimulating hormone for the evaluation of secondary hypothyroidism.

We performed a retrospective medical chart review of patients seen at a single institution as outpatients who had a simultaneously normal thyroid-stimulating hormone level, low FT₄ level, and any FT₄I measured between June 2014 and October 2016. Demographic, laboratory, and imaging data were collected. Using FT₄I as the reference for diagnosis of hypothyroidism, the sensitivity and specificity of the FT₄ immunoassay's lower-limit thresholds were determined. Within each threshold group, available brain imaging and biochemical evaluation were categorized according to the presence or absence of pituitary disease.

A total of 155 sets of result pairs (FT₄ and FT₄I) performed on 118 subjects were analyzed. The lower limit of a normal FT₄ level by immunoassay at this institution was 0.93 ng/dL, though all pairs with FT₄ ≥0.89 ng/dL had a normal FT₄I. All pairs with FT₄ ≤0.67 ng/dL had a low FT₄I. No pituitary macroadenomas were identified in any subject, though the rates of pituitary imaging in this patient sample were low.

Patients with a borderline low FT₄ level by immunoassay often have normal FT₄I. In such patients at our center, significant structural and biochemical pituitary pathology was uncommon.