Hypothyroidism in dogs

Symptoms an owner can spot at home

• Weight gain without any increase in food, the classic presenting complaint
• Lethargy, reduced stamina, and more sleeping
• Seeking heat: lying on vents, pressing against radiators, reluctance to go out in cold
• Symmetrical hair thinning over the flanks and tail ('rat tail'), with dull, dry coat and skin
• Recurrent skin or ear infections that keep coming back after treatment

When to see a vet

• Unexplained weight gain on a stable ration, this is a bloodwork question, not a diet question
• Symmetrical, non-itchy hair loss over both flanks
• Recurrent pyoderma or otitis that resolves on treatment and relapses within weeks
• New facial nerve deficits, head tilt, or weakness in a middle-aged dog
• Any dog on thyroid supplementation showing weight loss, panting, or restlessness, this can indicate over-supplementation

What it is

Over 95% of clinical canine hypothyroidism is primary: the thyroid gland itself fails, through lymphocytic thyroiditis (an immune-mediated attack on thyroid tissue) or idiopathic follicular atrophy. Thyroid hormone sets the basal metabolic rate of essentially every tissue, so the deficiency shows up as a slowdown across systems rather than as one dramatic sign.

Typical patients are middle-aged, medium to large breed dogs. Dermatologic and metabolic signs dominate. Less common presentations include neuromuscular deficits, corneal lipid deposits, and reproductive changes. Unlike cats, where the thyroid disease of note is hyperthyroidism, dogs almost always go the other direction.

How vets diagnose it

Diagnosis is not a single total T4 in isolation. Non-thyroidal illness and several drug classes suppress thyroid hormone levels in dogs that are not hypothyroid, a pattern called euthyroid sick syndrome, and treating it as hypothyroidism is a common misstep. Confirmation typically combines total T4, free T4 by equilibrium dialysis, and canine TSH, read against the clinical picture and a full chemistry panel.

• Supporting findings often include a mild non-regenerative anaemia and elevated cholesterol.
• Thyroglobulin autoantibody testing can support a diagnosis of lymphocytic thyroiditis.
• Sighthounds run naturally lower T4 reference values, breed-specific interpretation matters.

Treatment overview

This is editorial overview, not a treatment plan; your vet builds the plan. Management is lifelong thyroid hormone replacement, dosed by weight and then titrated to post-pill hormone concentrations rechecked at set intervals. Energy and demeanour usually improve within a few weeks; coat and skin changes take two to four months to fully reverse, so patience is part of the protocol.

Dosing consistency matters more than owners expect. Absorption is affected by timing relative to food and by concurrent supplements, particularly calcium and soy-containing products, so the routine set on day one should be the routine on day 300.

What owners can do

• Measure food by weight, not by scoop. Weight gain is the sign owners notice first and the variable they control most directly during titration.
• Recheck body condition score monthly. A written score beats a subjective impression when you see the dog every day.
• Keep the dog warm. Cold intolerance is real; a supportive, insulating or self-warming bed reduces the discomfort of lying on cold floors.
• Support the coat while it recovers: gentle regular brushing, fragrance-free shampoo, and vet-directed omega-3 supplementation.
• Give medication at the same time relative to meals every day, and tell your vet about every supplement in the house.
• Keep recheck appointments. Both under- and over-supplementation cause problems, and only bloodwork distinguishes them.