The clinical signs of hypothyroidism are quite variable, but can include some degree of mental dullness, lethargy, propensity for weight gain, and variable effects on the hair coat. Clinicopathologic abnormalities associated with hypothyroidism can include mild, normocytic, normochromic anemia, fasting hypercholesterolemia (present in up to 60% of dogs, but not pathognomonic for hypothyroidism), and occasional mild to moderate increases in serum ALT and ALP.
A serum total thyroxine (total T4) concentration in the mid-to-upper reference range rules out hypothyroidism. Although total T4 is low in hypothyroid dogs, it may also be reduced in dogs sick with other illness, or those receiving drug treatment, e.g., glucocorticoids, phenobarbital, sulfa-containing medications, or o,p,-DDD. Sight hounds can have lower total T4 values than other breeds. Falsely high values of serum total T4 may be seen in dogs with antibodies that interfere with some T4 assays- serum free T4 determination is indicated in such instances.
Serum free T4 is low in hypothyroid dogs and is thought to be less subject to decrease due to other illness or drug use.
The canine-specific endogenous TSH assay (cTSH) is increased (>0.7 nmol/L) in 60-70% of dogs with confirmed hypothyroidism.
Canine hypothyroidism can be difficult to definitively diagnose, particularly in the face of concurrent illness or drug therapy. In such circumstances, thyroid testing should only be used to rule-out disease. Current recommendations for testing at the AHL include a serum total T4 to rule out hypothyroidism, with serum free T4 and cTSH for further clarification of low total T4 results.
Therapeutic monitoring includes evaluation of the clinical response to thyroid hormone supplementation and measurement of serum total T4 concentrations. Serum T4 should be measured 4 to 8 wk after initiating therapy, whenever signs consistent with thyrotoxicosis develop, or when there has been minimal or no response to therapy. It should also be measured 2 to 4 wk after an adjustment in levothyroxine therapy.
Serum total T4 is typically measured 4 to 6 h after the administration of levothyroxine in dogs receiving either once or twice daily medication- this will assess the peak drug value.