Assessment of thyroid function in dogs with low plasma thyroxine concentration

Area of Science:

• Veterinary Internal Medicine
• Endocrinology
• Diagnostic Imaging

Background:

• Distinguishing canine hypothyroidism from nonthyroidal illness is difficult as plasma thyroxine (TT4) and thyroid-stimulating hormone (TSH) levels can be misleading.
• Low TT4 concentrations are common in both conditions, complicating diagnosis.

Purpose of the Study:

• To evaluate diagnostic methods for differentiating primary hypothyroidism from nonthyroidal illness in dogs.
• To assess the utility of various biochemical and stimulation tests, alongside thyroidal uptake measurements.

Main Methods:

• Analyzed plasma concentrations of TT4, freeT4 (fT4), and TSH in 30 dogs with low TT4.
• Evaluated TSH-stimulation tests, TSH-releasing hormone (TRH) stimulation, thyroglobulin antibodies (TgAbs), and quantitative thyroidal 99mTcO4(-) uptake.
• Final diagnoses were confirmed by histopathologic examination of thyroid tissue.

Main Results:

• Plasma TT4, fT4, TSH, and their ratios showed overlap between hypothyroid and nonthyroidal illness groups.
• TSH and TRH stimulation tests did not provide clear differentiation.
• Quantitative thyroidal 99mTcO4(-) uptake demonstrated no overlap, proving highly effective in distinguishing the two conditions.

Conclusions:

• Accurate biochemical diagnosis of primary hypothyroidism in dogs remains problematic.
• Previous diagnostic standards, like the TSH-stimulation test, may lead to misclassification.
• Quantitative thyroidal 99mTcO4(-) uptake is the most powerful tool for differentiating canine primary hypothyroidism from nonthyroidal illness.