Pituitary disease unlikely in adults with low free T4 and no
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Most adults with free thyroxine levels slightly below the reference range have normal free T4 index values and did not have abnormal findings on pituitary imaging or functional tests, according to a study published in Endocrine Practice.

This study aimed 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.

Researchers 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. 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.

Results
-- 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 more than 0.89 ng/dL had a normal FT4I.

-- All pairs with FT4 less than 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.

Conclusively, 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.

Source: https://www.endocrinepractice.org/article/S1530-891X(20)48326-0/fulltext
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