The existence of hypothyroidism in greyhounds remains controversial and its investigation is complicated by the low circulating thyroid hormone concentrations typically found in healthy dogs of this breed. Quantitative measurement of thyroidal technetium-99m pertechnetate (99mTcO₄-) uptake is known to be useful in accurately assessing thyroid function as it is less likely to be affected by the myriad extrathyroidal factors capable of influencing thyroid hormone concentrations. The aim of this study was to evaluate thyroid scintigraphy as a means of assessing thyroid function in greyhounds.

The study comprised 14 entire (8 female, 6 male) greyhounds ranging in age from 16-66 months. All were presented for investigation of clinical signs (hair loss, poor performance) that the owners/trainers considered commensurate with a need for thyroid hormone supplementation. Jugular blood samples were obtained and total T4 and cTSH concentrations measured by commercial chemiluminescent assays (Immulite 1000 total T4 and canine TSH, respectively, DPC). All total T4 concentrations (median 6.44 nmol/L) were below while cTSH concentrations (median 0.14 ng/mL) were within the standard reference limits (15-50 nmol/L; < 0.68 ng/mL, respectively).

Approximately 140-160 MBq 99mTcO₄- was injected intravenously and 45 minutes later ventral images of the head and neck were acquired using a gamma camera (General Electric 400) fitted with a low energy, all purpose parallel hole collimator and percent uptake calculated using Hermes Gold 2.1 software. In all dogs, both thyroid lobes were visible and the mean value was used for subsequent analyses. All values (mean ± sd (range), 0.75 ± 0.29 (0.40-1.49) %) were within the reference limits published for euthyroid dogs (0.39-1.86 %). Thyroid:salivary gland ratios were also calculated but provided limited information on thyroid function because of variable salivary uptake noted in dogs of this study.

This study confirms that thyroid scintigraphy provides valuable information on thyroid function in greyhounds and can accurately depict euthyroidism despite low circulating total T4 values. However, calculation of percent thyroidal uptake of 99mTcO₄- appears to be more accurate than thyroid:salivary gland ratios and should be used when investigating thyroid function in this breed.