Quantitative Thyroid Scintigraphy in Greyhounds Suspected of Primary Hypothyroidism

Abstract

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 ((99m)TcO4-) uptake is known to be useful in assessing thyroid function in other breeds. The aim of this study was to evaluate thyroid scintigraphy as a method of assessing thyroid function in greyhounds suspected of primary hypothyroidism. Twenty greyhounds (eight females, 12 males) were studied. Thirteen had bald thigh syndrome and seven poor performance and low total T4. Total T4 concentrations were decreased in 18 (90%), and free T4 in two (10%) dogs. All canine thyroid stimulating hormone concentrations were within the reference interval. Thyroidal (99m)TcO4- uptake values (mean +/- SD, 0.76 +/- 0.26%) were within the reference limits published for euthyroid dogs (0.39-1.86%) making hypothyroidism highly unlikely. There were no significant differences (P < 0.05) when comparing data between dogs with bald thigh syndrome (13 dogs) and the remaining dogs (seven dogs). Seventeen (85%) dogs had higher uptake in the left thyroid gland than in the right that might reflect an anatomic feature of the greyhound breed. Calculation of percent thyroidal uptake of (99m)TcO4- is more accurate than thyroid: salivary gland ratios because of high variability in salivary gland uptake. Percent thyroidal uptake of (99m)TcO4- should be used when assessing thyroid function scintigraphically in the greyhound breed.

Quantitative Pertechnetate Thyroid Scintigraphy and the Ultrasonographic Appearance of the Thyroid Gland in Clinically Normal Horses

“…There were also no significant differences when comparing data between dogs with bald thigh syndrome and those without. 79 Thyroidal percent dose uptakes for the greyhounds in this study were higher than those previously reported in the literature for dogs with primary hypothyroidism (0.03-0.26%), 64 suggesting the greyhounds included in the study were not hypothyroid.…”

“…80 unclear but may represent a normal finding in this breed. 79 Salivary percent dose uptake was between 0.45% and 2.1%. T/S ratios were between 0.65 and 2.36.…”

“…64,79 One of these studies focused on using scintigraphy to differentiate between dogs with primary hypothyroidism and those with low thyroid hormone concentrations secondary to non-thyroidal illness. 64 Medical conditions reported to decrease total T 4 concentrations in dogs include hyperadrenocorticism, diabetic ketoacidosis, hypoadrenocorticism, renal failure, hepatic disease, peripheral neuropathy, megaesophagus, heart failure, critical illness or infection and surgery or anesthesia.…”

“…Due to variability in salivary uptake it would be more reliable to use thyroidal percent dose uptake than T/S ratio when assessing thyroid gland function in dogs. 79 Sonography in evaluation of the thyroid gland…”

“…In dogs, percent dose uptake of pertechnetate by the thyroid gland is thought to be a more reliable parameter than T/S ratio when assessing thyroid function. 79 Percent dose uptake of pertechnetate is a more difficult parameter to calculate and is not affected by salivary uptake of pertechnetate or secretion of saliva. Pertechnetate scintigraphy is used in dogs to investigate the origin of cervical masses and can be used to diagnose hypothyroidism.…”