Dog Hypothyroidism Signs: Weight Gain, Coat & Energy
Hypothyroidism is the most common hormonal disorder in dogs, caused by an underactive thyroid gland producing insufficient thyroid hormone. It progresses slowly and is easily confused with normal aging, but it is diagnosable with a blood test and treatable with inexpensive daily medication. Early diagnosis prevents months of unnecessary suffering.
Last reviewed: June 2026
What Is Dog Hypothyroidism?
The thyroid gland produces thyroxine (T4), which regulates metabolism, energy, body temperature, coat health, and organ function throughout the body. Hypothyroidism occurs when the thyroid produces too little T4 — in 95% of dogs this results from immune-mediated lymphocytic thyroiditis or idiopathic follicular atrophy, both of which destroy thyroid tissue over time. The condition is most common in middle-aged dogs (4–10 years) and has a known breed predisposition, as described in Behrend et al., 2013, JVIM, the ACVIM consensus that established diagnostic and treatment standards for canine endocrine disorders.
Signs of Hypothyroidism in Dogs
Because thyroid hormone affects every organ system, hypothyroidism produces a wide, non-specific range of signs. The most consistently reported include:
- Weight gain without any increase in food intake
- Lethargy and exercise intolerance disproportionate to age
- Cold intolerance — seeking warmth, shivering in normal temperatures
- Coat and skin changes: dry, brittle coat; diffuse hair loss (especially over the trunk and tail base, often symmetrical); thickened, puffy skin ("tragic facial expression"); failure of hair to regrow after clipping
- Bradycardia (slow heart rate) detectable on examination
- Neurological signs in severe or long-standing cases: peripheral neuropathy, facial nerve paralysis, or vestibular signs
Signs build over 6–24 months, which is why owners often attribute them to normal aging, as described in the AAHA Canine Life Stage Guidelines, 2019. Any middle-aged dog with symmetrical hair loss plus unexplained weight gain should be screened.
Breeds at Higher Risk
Golden Retrievers, Doberman Pinschers, Irish Setters, Boxers, Cocker Spaniels, and Airedale Terriers are consistently over-represented in hypothyroidism case series, as described in Ettinger's Textbook of Veterinary Internal Medicine. The condition is rare in cats (feline hypothyroidism almost exclusively occurs after radioiodine therapy for hyperthyroidism).
Diagnosis
The screening test is a baseline total T4 (TT4). A low TT4 in a dog with compatible signs is highly supportive. Because illness or medications can suppress TT4 in euthyroid dogs ("sick euthyroid syndrome"), equivocal cases require free T4 measured by equilibrium dialysis (fT4 ED) — the gold standard. Endogenous TSH (cTSH) elevation with low fT4 ED confirms primary hypothyroidism. As described in Behrend et al., the combination of low fT4 ED plus elevated cTSH provides the highest diagnostic specificity.
Treatment
Levothyroxine (synthetic T4) administered orally once or twice daily is the treatment. Most dogs show improvement in energy, coat, and weight within 4–8 weeks of starting therapy. The dose is adjusted based on 4–6 hour post-pill T4 monitoring at the 4–8 week recheck. Dogs require lifelong treatment and twice-yearly monitoring once stabilized. Levothyroxine is inexpensive — generic formulations cost $20–50/month for most dog sizes.
When to See a Vet
Call your vet today if:
- Your middle-aged dog has gained significant weight without eating more
- You notice symmetrical hair loss over the back, flanks, or tail base
- Your dog seems persistently lethargic or cold-seeking
- Coat has become dry, dull, or stopped regrowing after clipping
Go to the ER immediately if:
- Your dog collapses, is unresponsive, or has an extremely slow heart rate (myxedema coma — a rare, severe hypothyroid emergency)
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Frequently Asked Questions
What blood test diagnoses hypothyroidism in dogs?
The initial screening test is baseline total T4 (TT4). If results are equivocal, free T4 by equilibrium dialysis (fT4 ED) combined with endogenous TSH (cTSH) provides the most accurate diagnosis. A full thyroid panel runs $100–250 at most veterinary labs, either in-house or sent to a reference laboratory.
How much does treating hypothyroidism in dogs cost?
Initial thyroid bloodwork costs $80–200. Levothyroxine medication runs $20–50/month at generic pricing. Twice-yearly recheck blood tests cost $80–150 each. The condition is among the most affordable chronic diseases to manage once diagnosed — costs are primarily ongoing monitoring rather than expensive treatment.
Can hypothyroidism be confused with Cushing's disease in dogs?
Yes — both cause weight gain, lethargy, and coat changes, and occasionally occur together. Key differentiators: Cushing's causes a pot-bellied appearance, increased thirst and urination, and elevated liver enzymes, while hypothyroidism causes cold intolerance and a slow heart rate. A full endocrine workup distinguishes the two.
How long does it take to see improvement after starting levothyroxine?
Most dogs show measurable improvement in energy level within 2–4 weeks. Coat regrowth takes longer — significant improvement is usually visible by 8–12 weeks. Weight normalization follows over 3–6 months with appropriate dietary management. Regular monitoring at 4–8 weeks post-initiation confirms the dose is correct.
Will my dog need thyroid medication forever?
Hypothyroidism in dogs is almost always permanent because the thyroid tissue has been irreversibly damaged or destroyed. Lifelong daily medication is required. Fortunately, levothyroxine is safe, effective, and well-tolerated with minimal side effects when properly dosed and monitored.
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