Dr. W Jean Dodds is an international thyroid expert and leads the clinical research team at Hemopet’s Hemolife Diagnostics. Hypothyroidism is the most common endocrine disorder in dogs, and yet is one of the most challenging conditions to diagnose. We thought we would provide a brief overview of the thyroid gland, testing protocols and new discoveries she and the team have made. For more in-depth information, please read her award-winning book, written with Diana Laverdure, The Canine Thyroid Epidemic: Answers You Need for Your Dog, (2011, Dog Wise).
The thyroid gland is part of the endocrine system, which controls production of the body’s hormones. The thyroid is a delicate gland located in the upper third of the neck. 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 in order for cellular activities to function properly.
• 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 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 autoimmune thyroiditis.
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. 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.
* Dr. Dodds and her team have submitted the more detailed findings for formal, refereed publication.
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.