Immune-Mediated Thyroiditis in Yorkshire Terriers

Hypothyroid signs are non-specific in any dog: lethargy, weight gain without more food, cold intolerance, a dull or thinning coat, recurring skin problems, sometimes a change in mood or reactivity. In a large working breed a drop in energy is conspicuous. In a 4-to-7-pound Yorkshire Terrier with a documented exercise need of two out of five, it simply is not.

The coat is often the more honest signal in this breed, because a Yorkie's long, silky coat is a big part of why owners look at their dog closely every day. Coat that thins, goes dry or brittle, loses its shine, or grows back slowly after clipping is worth taking seriously. So is skin that suddenly becomes prone to infections when it never used to be.

Weight is the other one, and it is deceptive at this size. On a seven-pound dog, half a pound is a substantial proportional gain and is nearly invisible under coat. Weighing monthly on a kitchen scale catches what looking at the dog does not.

There are two different questions and they get conflated. The first is whether the thyroid is underperforming — that is a hormone question, answered by measuring thyroid hormone levels and, importantly, by a full panel rather than a single total T4 in isolation, because a lone low T4 can be produced by other illness entirely.

The second question is why. Immune-mediated thyroiditis is identified by looking for thyroglobulin autoantibodies — evidence that the immune system is actively targeting thyroid tissue. A dog can carry those antibodies while hormone levels are still normal, which means the process is underway but the gland is still compensating.

That second finding is genuinely useful information rather than academic. It tells you this is likely to progress, it justifies monitoring rather than waiting for symptoms, and in a breeding context it matters a great deal. Ask your vet specifically what was measured, because 'the thyroid test was fine' can mean quite different things depending on which test was run.