Hypothyroidism
A common hormonal disorder in dogs where an underactive thyroid slows metabolism, causing weight gain, low energy, and coat changes.
Also known as Underactive Thyroid, Low Thyroid, Thyroid Deficiency
Hypothyroidism happens when the thyroid gland, a small structure in the neck, stops producing enough thyroid hormone to keep the body's metabolism running normally. In dogs, this is almost always because the gland itself has been damaged, usually by the immune system mistakenly attacking it over time, or because the tissue gradually shrinks and stops functioning for reasons that aren't fully understood. It's one of the most frequently diagnosed hormonal conditions in dogs, and while it can occur in cats too, that's rare unless it happens as a side effect of treating an overactive thyroid.
Because thyroid hormone affects nearly every system in the body, the signs tend to creep in slowly and can look like ordinary aging or simple weight gain at first. Owners often notice their dog seems more tired than usual, has slowed down on walks, gains weight without eating more, or seems to seek out warm spots more than before. The most common outward signs are actually skin and coat related: thinning hair, especially over the back and tail, a dull or brittle coat, darkened or thickened skin, and repeated skin infections. Middle aged, medium to large breed dogs are affected most often, with golden retrievers, Doberman pinschers, and beagles among the breeds that turn up more frequently in cases.
What makes it tricky to diagnose
None of these signs point specifically to the thyroid on their own, which is part of why this condition has a reputation for being both under and overdiagnosed. A vet will usually start with a screening blood test measuring total T4, one of the main thyroid hormones, then follow up with additional tests such as free T4 and TSH if the result is low, since several other illnesses and even some medications can push thyroid readings down temporarily without the thyroid actually being at fault. Diagnosis really depends on putting the blood test results together with the dog's history, breed, and physical exam findings rather than relying on a single number.
The reassuring part is that once properly diagnosed, hypothyroidism is very manageable. Treatment involves a daily synthetic hormone tablet called levothyroxine, taken for the rest of the dog's life, which replaces what the thyroid can no longer make on its own. Most dogs respond well once their dose is fine tuned, though getting there usually takes a follow up blood test about a month after starting treatment, and periodic rechecks afterward since the right dose can shift over time. Left untreated, the condition doesn't tend to be an emergency, but it steadily wears on quality of life and can contribute to skin problems, sluggish heart function, and in rare advanced cases, nerve or balance issues. If your pet is showing a mix of unexplained weight gain, low energy, and coat changes, it's worth a conversation with your vet rather than chalking it up to getting older.
