The thyroid gland is part of the endocrine system, which controls production of the body’s hormones. The thyroid is a delicate butterfly-shaped gland located in the upper third of the neck in front of the trachea (windpipe). It takes iodine supplied in food and uses it to produce the hormones T4 (thyroxine) and T3. These hormones are used by the body to control metabolism. Even though iodine is required for this process, either a deficiency or an excess of iodine can profoundly affect thyroid function and promote autoimmune thyroiditis (the heritable form of hypothyroidism).
The thyroid gland is an essential gland. If it is diseased, destroyed, or surgically removed, thyroxine hormone replacement therapy must be provided for cellular activities to function properly.
Testing Measures
- Total T4 – Total amount of T4 (thyroxine) hormone that circulates in the blood. More than 99% of T4 hormone is “bound”, which means that it attaches to proteins in the blood and never reaches the tissues.
- Free T4 – Serum free T4 represents the less than 0.1% of thyroxine hormone that is unbound and biologically active. The blood’s free T4 level tells the pituitary gland whether or not it needs to produce more Thyroid Stimulating Hormone (TSH).
- Total T3 – Similar to total T4, total T3 represents both the bound and unbound forms of T3 circulating in the blood. Measuring serum T3 alone is not considered an accurate method of diagnosing canine thyroid disorder, as this hormone reflects tissue thyroid activity and is often influenced by concurrent primary or secondary non-thyroidal illness (NTI), also termed “euthyroid-sick syndrome”. NTI is a state of adaptation where the levels of T3 and/or T4 are at unusual levels, but the thyroid gland does not appear to be dysfunctional. This condition is often seen in starvation, critical illness, or patients in intensive care unit.
- Free T3 – Less than 0.1% of T3 molecules circulate freely in the blood and are biologically active. The blood’s free T3 level also tells the pituitary gland whether or not it needs to produce more Thyroid Stimulating Hormone (TSH).
- Canine Thyroglobulin Autoantibody (TgAA) – Elevated TgAA is present in the blood of dogs with heritable autoimmune thyroiditis, like human Hashimoto’s disease.
- Note: Unlike humans where the human-specific TSH test is 95% predictive of thyroid function, in dogs, the canine-specific TSH test is only ~70% predictive of thyroid status because dogs have a more active thyroid regulatory pathway via growth hormone.
Discoveries – T4:FT4 Ratio
Previously, it was often difficult to distinguish between actual hypothyroid dogs and those who have NTI. Dr. Dodds and her clinical research team at Hemopet’s Hemolife Diagnostics have established and patented a more accurate diagnostic method called the T4:FT4 Ratio, which is the ratio between the level of total T4 (thyroxine) and its free, unbound fraction. This provides enhanced diagnostic specificity for canine thyroid disorders. Importantly, veterinarians can now determine whether the values of the different thyroid analytes being measured indicate a thyroid disorder or some other underlying disease. Note that this ratio does not diagnose or rule out the presence of heritable autoimmune thyroiditis, where affected dogs have elevated thyroid autoantibodies (typically high TgAA).
Statistical Research Confirming the Importance of the T4:FT4 Ratio
1000 healthy dogs had a tight T4:FT4 ratio ranging from 1.30-1.54.
Ratios <1.25 indicated the presence of primary NTI or NTI secondary to hypothyroidism. A subset of 242 dogs with elevated liver enzymes (high ALT) was further analyzed as a group confirmed to have NTI. Of these, 119 dogs were receiving thyroxine treatment for hypothyroidism.
Determination: The T4:FT4 Ratio was determined to be a critical factor in distinguishing dogs with primary hypothyroidism from those with primary or secondary NTI. The results were highly statistically significant (p=< 0.001). Further, the variance of T4 data for the 223 dogs with liver disease not receiving thyroxine was much higher than that of the 119 dogs with high ALT that had their hypothyroidism corrected by thyroid medication (2.225 vs 0.085).
This once again validates the need for doing comprehensive thyroid panels and the special importance of the T4 and free T4 data, along with measuring TgAA to exclude thyroiditis. Without this information, a disorder can be misdiagnosed or valuable effort and time expended on inaccurate care and treatment.
T4:FT4 Ratio Patent
Hemopet is the only diagnostic laboratory in the world that can use and provide the T4:FT4 Ratio and findings. Using the Hemopet’s all “green” technology and breed- and age-specific interpretive analysis, plus the T4:FT4 Ratio, veterinarians and clients are once more assured of obtaining the leading thyroid technology profiles and analysis.
To obtain a copy of Dr. W. Jean Dodds’ book The Canine Thyroid Epidemic, please visit Amazon.