Canine Hypothyroidism: NAVLE Study Guide

Hypothyroidism is the most common endocrine disorder in dogs, and it shows up on the NAVLE constantly. The exam loves to test breed recognition, the TT4/fT4/cTSH triad, and the euthyroid sick syndrome trap. Get this one right and you pick up easy points.

What Actually Causes It

Two diseases account for nearly all cases. Lymphocytic thyroiditis (immune-mediated) destroys thyroid follicles over time – dogs often have detectable thyroglobulin autoantibodies years before clinical signs appear. Idiopathic follicular atrophy is the other major cause; the follicles just waste away with no immune component. Both produce the same result: inadequate T4 and T3 synthesis, with the pituitary compensating by cranking out TSH. That’s the pattern to recognize.

Secondary hypothyroidism (pituitary TSH deficiency) exists but is rare in dogs. If you see a question with low T4 AND low TSH, think secondary – or think euthyroid sick syndrome first, because that’s far more common.