Biological variation of serum thyrotropin and thyroid hormones concentrations determined at 8-week intervals for 1 year in clinically healthy cats

Abstract

Background: Cats commonly develop thyroid disease but little is known about the long-term biological variability of serum thyroid hormone and thyrotropin (thyroid-stimulating hormone; TSH) concentrations.

Objectives: We aimed to determine the long-term biological variation of thyroid hormones and TSH in clinically healthy cats.

Methods: A prospective, observational study was carried out. Serum samples for analysis of total thyroxine (T_4, by radioimmunoassay [RIA] and homogenous enzyme immunoassay [EIA]), triiodothyronine (T₃ ), free T₄ (by dialysis), and TSH were obtained every 8 weeks for 1 year from 15 healthy cats, then frozen until single-batch analysis. Coefficients of variation (CV) within individual cats ( ${\mathrm{CV}}_I$ ) and among individual cats ( ${\mathrm{CV}}_G$ ), as well as the variation between duplicates (ie, analytical variation [ ${\mathrm{CV}}_A$ ]) were determined with restricted maximum likelihood estimation. The indices of individuality (IoI) and reference change values (RCVs) for each hormone were calculated.

Results: Some thyroid hormones showed similar (total T₄ by EIA) or greater (TSH) interindividual relative to intraindividual variation resulting in intermediate to high IoI, consistent with previous studies evaluating the biological variation of these hormones weekly for 5-6 weeks. By contrast, total T₄ (by RIA) and free T₄ had a low IoI. Total T₃ had a high ratio of ${\mathrm{CV}}_A$ to ${\mathrm{CV}}_I$ ; therefore, interindividual variation could not be distinguished from analytical variation. No seasonal variability in the hormones could be demonstrated.

Conclusions: Clinicians might improve the diagnosis of feline thyroid disease by establishing baseline concentrations for analytes with intermediate-high IoI (total T_4, TSH) for individual cats and applying RCVs to subsequent measurements.

Keywords: index of individuality; population-based reference intervals; reference change value; subject-based reference intervals; thyroid-stimulating hormone; thyroxine; triiodothyronine.