Hypothyroidism in Dogs: Coat, Weight and Mood Change First

The coat goes first. Not dramatically, not all at once, but in the way a garden thins at the edges before the centre gives way. A dog that once had a full, glossy coat starts shedding more than usual, then grows it back more slowly, then stops growing it back in certain patches entirely. The skin underneath may look thickened, almost leathery. This is often the first signal that the thyroid gland, a small butterfly-shaped gland in the neck, has begun underproducing the hormones that regulate nearly every metabolic process in the body.

Thyroxine (T4) and triiodothyronine (T3) govern how fast cells burn energy. In a dog with hypothyroidism, the gland produces too little of both. The result is a system-wide slowdown. Metabolism drops, meaning the dog burns fewer calories at rest. The skin and hair follicles, which are among the most metabolically active tissues in the body, are among the first to show the deficit. Hair growth cycles slow, follicles miniaturise, and the coat becomes sparse, dull, and brittle. Bilateral symmetry is a classic tell: the thinning often appears on both flanks at the same time, or along the tail and the bridge of the nose, because those follicles are under the same hormonal pressure simultaneously.

A dog gaining weight despite no change in diet or activity is one of the clearest early signs of hypothyroidism. Because the metabolic rate has dropped, the same number of calories that once maintained a healthy weight now creates a surplus. The weight tends to distribute evenly rather than accumulating in one area, and it does not respond to modest reductions in food the way ordinary weight gain does. Owners often blame themselves, cutting portions, switching to a lighter formula, adding walks, and watching the number on the scale continue to climb. The thyroid is the more likely explanation when the gain is persistent, steady, and resistant to the usual corrections. In some dogs, a puffy, thickened appearance develops around the face, a condition called myxoedema, caused by the accumulation of certain proteins under the skin when metabolism is suppressed.

A hypothyroid dog often becomes quieter, slower to engage, and less interested in play or walks. Owners frequently attribute this to the dog getting older, especially in middle-aged dogs between four and ten years, which is the typical window for hypothyroidism to develop. But there is a difference between a dog that is slowing down naturally and one whose neurological function is being suppressed by low thyroid hormone. Hypothyroid dogs may sleep more than usual, startle less, and seem mentally foggy, slower to respond to commands they know well, less curious about the environment. In some cases, peripheral neuropathy develops, causing a subtle change in gait or a reluctance to climb stairs. These signs, taken together with the coat and weight changes, form a pattern that is worth investigating rather than accepting as inevitable.

Hypothyroidism is more common in medium to large breeds. Golden Retrievers, Dobermann Pinschers, Irish Setters, Boxers, Cocker Spaniels, and Labrador Retrievers appear more frequently in the literature than small or toy breeds. The condition is rare in cats and uncommon in very small dogs. If your dog fits the breed profile and is showing two or more of the classic signs, symmetrical coat thinning, unexplained weight gain, and a shift toward lethargy, a thyroid panel is a straightforward next step. The test measures total T4 and, in ambiguous cases, free T4 and thyroid-stimulating hormone (TSH). A single low T4 reading is not always conclusive on its own, since other illnesses can suppress thyroid levels temporarily; a veterinarian will read the numbers alongside the clinical picture. Treatment, where hypothyroidism is confirmed, is typically a daily oral synthetic hormone replacement that most dogs tolerate well, with coat and energy improvements often visible within several weeks.

The coat is usually the first thing owners notice recovering. Hair begins filling back in at the flanks and tail within a couple of months of starting treatment, and the skin texture normalises as the follicles return to a normal growth cycle. Weight follows more slowly, because the metabolic shift takes longer to reverse and because some of the gain may have been compounded by reduced activity over months. The temperament change tends to be the most striking for owners: dogs that had become quiet and disengaged often become noticeably more alert and interactive within weeks of reaching a stable hormone level. Monitoring requires periodic bloodwork to keep the replacement dose calibrated, since the correct dose can shift as the dog ages or changes weight. The condition is managed rather than cured, but managed well, most dogs return to something close to their previous quality of life.