A pilot study: subclinical hypothyroidism and free thyroid hormone measurement by immunoassay and mass spectrometry

The diagnosis of subclinical hypothyroidism is defined as the presence of an elevated thyroid stimulating hormone (TSH) with a normal free thyroxine (FT4) level. The commonly used direct analogue immunoassays for the measurement of FT4 have been shown to have poor performance at the upper and lower limits of the FT4 reference interval.

The purpose of this pilot study was to investigate the percentage of individuals classified as having subclinical hypothyroidism with a standard immunoassay, that actually have low free thyroid hormone levels by mass spectrometry measurements.

Outpatient samples with elevated TSH values and normal FT4 concentrations as per standard immunoassay methods were collected. FT4 and free triiodothyronine (FT3) analyses were performed on these samples using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

Sixty five percent (n=26) of patients (n=40) had (LC-MS/MS) FT4 or FT3 or both FT4 and FT3 values below mass spectrometry reference limits.

Our findings indicate that the direct analogue immunoassay method for FT4 measurement results in a significant proportion of patients being misclassified as having subclinical hypothyroidism.