Diagnosis of Canine Hypothyroidism
Staci Goussev, DVM, DACVIM
Introduction
Acquired primary hypothyroidism is the most common cause of naturally occurring thyroid failure in the adult dog, accounting for more than 95% of cases. This accounts for the majority of cases seen at DoveLewis. Thyroid hormones regulate many metabolic processes, and no tissue or organ system escapes the adverse effects of thyroid hormone insufficiency. These effects can be far ranging and include: fetal development, all aspects of carbohydrate and lipid metabolism, cardiac contractility, stimulation of erythropoiesis, respiratory center effects, and facilitation of the response to catecholamines.
Common Clinical Manifestations of Hypothyroidism
Alterations in the skin and hair coat are the most commonly observed abnormalities and are present 60-80% of the time. A normocytic, normochromic, nonregenerative anemia is identified in approximately 30% of dogs. The cause is unknown but is believed to be due to decreased erythrocyte production. The classic abnormality seen on a screening biochemistry panel is fasting hypercholesterolemia, which is present in approximately 75% of dogs. Fasting hypertriglyceridemia is also very common.
Alopecia in area of wear, under collar.
Common symptoms include heat seeking behavior and endocrine alopecia in areas of friction or wear.
Example of myxedema, or “Tragic Face”.
Testing and Interpretation
T4 is the major secretory product of the normal thyroid gland. Thyroid hormones in plasma are highly protein bound with T4 more highly bound than T3. Less than 1% of T4 and T3 circulate in the unbound “free” state. Only free or unbound thyroid hormones enter cells to produce a biologic effect or regulate pituitary TSH secretion.
Baseline serum total T4 concentration is the sum of both protein bound and free hormone circulating in the blood. Measurement of the serum T4 concentration can be used as the initial screening test for hypothyroidism and theoretically is straightforward as a rule out test: if T4 is normal, then the pet is not hypothyroid.
However, the case of a low T4 is not always straightforward. Many physiologic and pharmacologic factors (such as phenobarbital and steroids) affect the pituitary-thyroid axis and interfere with the accuracy of baseline serum T4 concentration for differentiating hypothyroidism from euthyroidism. This can lead to overlapping values between hypothyroid dogs and dogs with euthyroid dogs with non-thyroidal illness.
In situations of concern for hypothyroidism versus euthyroid dogs with non-thyroidal illness, evaluation of complete blood count and chemistry panel to determine if other markers of hypothyroidism are present (anemia, hypercholesterolemia) can be helpful. In addition, assessment of a free T4, or non-protein bound biologically active form of thyroid hormone, by equilibrium dialysis is an easy test to perform and can help to differentiate between sick euthyroid syndrome and true hypothyroidism. Another test that can be a beneficial piece of the puzzle for hypothyroidism diagnostics is canine TSH measurement.
Algorithm for Hypothyroid Testing
Treatment
Thankfully, treatment is relatively straightforward and involves supplementation of thyroid hormone. The recommended starting dose of Levothyroxine is 0.02mg/kg PO every 12 hours. Recheck is recommended in 6-8 weeks with a T4 measurement at 4-6 hours post the morning dose of levothyroxine. Based on this result, the dose can be titrated.
Conclusion
The diagnosis of thyroid disease can be intimidating and confusing due to the phenomenon of sick euthyroid syndrome, which falsely lowers T4 values with non-thyroidal illness. However, by keeping in mind the common clinical manifestations as outlined above, and staying alert to hypercholesterolemia and a normocytic, normochromic, nonregenerative anemia in middle aged to older dogs, your clinical suspicion can be heightened. In addition, evaluation of a total T4, followed by further evaluation of free T4 and TSH levels if needed will put you on the right track to a true diagnosis of hypothyroidism.
References
- Mooney, C. T. (2017). Canine Hypothyroidism. In S. J. Ettinger, E. C. Feldman & E. Cote (Eds), Textbook of Veterinary Internal Medicine, 8th Edition (pp. 1731-1742). Elsevier.
- Scott-Moncrieff, J. C. (2015). Hypothyroidism. In Canine & Feline Endocrinology, 4th Edition (pp. 77-135). Elsevier.