Hypothyroidism is the most common endocrine disorder of canines, and up to 80% of cases are believed to result from autoimmune (lymphocytic) thyroiditis. It takes destruction of at least 75% of the thyroid gland by targeted T-lymphocytes, before classical clinical signs of hypothyroidism are manifested. Thus, accurate diagnosis of the early compensatory stages of canine autoimmune thyroiditis that lead up to hypothyroidism affords important genetic and clinical options for prompt intervention and case management. The heritable nature of this disorder poses significant genetic implications for breeding stock.

Despite the fact that thyroid dysfunction is the most frequently recognized endocrine disorder of pet animals, it is often difficult to make a definitive diagnosis. As the thyroid gland regulates metabolism of all body cellular functions, reduced thyroid function can produce a wide range of clinical manifestations, sometimes vague, other times classical, and occasionally very unusual. Many of these clinical signs mimic those resulting from other causes and so recognition of the condition and interpretation of thyroid function tests can be problematic. Further, development of thyroid dysfunction is a continuum that begins with normalcy and progresses gradually over months to several years to end-stage disease.

A complete baseline thyroid profile should be measured and typically includes total T4, free T4, thyroid autoantibodies, and may also include total T3, free T3 and cTSH. If included in thyroid profiles, the T3 and freeT3 assays usually reflect thyroiditis when both are spuriously elevated due to presence of T3 autoantibody. The autoantibody (AA) assays (T3AA, T4AA, TgAA) are especially important in screening breeding stock for heritable autoimmune thyroid disease.

The normal reference ranges for thyroid analytes of healthy adult animals tend to be similar for most breeds of companion animals. Exceptions are the sighthound and giant breeds of dogs which have lower basal levels. Typical thyroid levels for healthy sighthounds, such as retired racing greyhounds, are at or just below the established laboratory reference ranges, whereas healthy giant breeds have optimal levels between the lower end and midpoint of these ranges.

Further, because young animals are still growing and adolescents are maturing, optimal thyroid levels are expected to be in the upper half of the references ranges. For geriatric animals, basal metabolism is usually slowing down, and so optimal thyroid levels are likely to be closer to midrange or even slightly lower.

All animals are not the same

•  Puppies have higher basal thyroid levels than adults
  
•  Geriatrics have lower basal thyroid levels than adults
  
•  Large/giant breeds have lower basal thyroid levels
  
•  Sighthounds have much lower basal thyroid levels