Hypothyroidism

Hypothyroidism occurs when the thyroid glands - located in the neck - produce insufficient thyroid hormone. Thyroid hormones regulate metabolism across every body system, so deficiency causes wide-ranging effects. It is the most common hormonal disorder in dogs, typically affecting middle-aged to older animals.

The condition develops gradually, and the signs are often attributed to normal ageing until a blood test confirms the diagnosis. Once diagnosed, it is highly manageable with daily oral thyroid supplementation (levothyroxine).

What to monitor

Because hypothyroidism develops slowly and the signs are non-specific, baseline monitoring before and after treatment provides the clearest picture.

Body weight. Weight gain is one of the most consistent signs. Monthly weighing establishes the trend.

Energy level. Lethargy, reduced exercise tolerance, and reluctance to play or engage are common. Log weekly energy on a simple scale.

Coat and skin condition. Dry, dull, or brittle coat; hair loss (often symmetrical on the body but not the head or limbs); skin that is thickened, scaly, or prone to infection. Note these changes with dates and describe what you're seeing specifically.

Cold intolerance. Dogs with hypothyroidism often seek warmth and are uncomfortable in cool temperatures.

Facial changes. Some dogs develop a "tragic" facial expression due to thickening of facial skin - myxoedema. The brow appears furrowed.

Heart rate. Bradycardia (slow heart rate) is a less common sign but worth noting if your vet finds it at examination.

After starting treatment: note changes in all of the above. Most dogs show visible improvement within 4–8 weeks of starting the correct levothyroxine dose. Energy returns, coat improves, and weight stabilises. Logging these changes confirms the treatment is working.

Treatment monitoring

Levothyroxine is given once or twice daily; your vet will advise on timing relative to meals (absorption varies with food). Blood monitoring is required:

• 4–8 weeks after starting treatment or changing dose - to measure T4 levels and confirm the dose is appropriate
• Every 6 months once stable

The target is a T4 level in the upper half of the normal reference range, measured at a consistent time relative to the medication dose. Bring your log of when medication was given to blood test appointments.

Consistency matters

Levothyroxine requires consistent dosing at consistent times. Missing doses allows T4 levels to drop and symptoms to return. Log each dose given, noting any missed doses and why.

Some medications, supplements, and foods affect levothyroxine absorption - calcium, antacids, and certain heart medications are common examples. Discuss any other medications with your vet when levothyroxine is started.

When to contact your vet

Contact your vet soon if:

• Your dog is showing signs consistent with hypothyroidism (weight gain, lethargy, coat changes) and hasn't been tested
• Your dog has been diagnosed and isn't responding to treatment after 8 weeks
• Signs of hypothyroidism have returned in a dog previously well-controlled

Contact your vet for a routine appointment to:

• Schedule the 4–8 week post-treatment blood check
• Review the 6-monthly T4 monitoring
• Discuss any changes in other medications that might affect levothyroxine

Frequently Asked Questions

How is hypothyroidism diagnosed? A blood test measuring total T4 (thyroxine) is the standard screen. Because T4 can be suppressed by other illnesses ("sick euthyroid syndrome"), a low result in an unwell dog may be falsely low. If the picture is unclear, free T4 measurement or a TSH (thyroid stimulating hormone) test may be added.

Does my dog need thyroid supplementation for life? Almost certainly, yes. Hypothyroidism in dogs is nearly always permanent - it results from immune-mediated destruction of thyroid tissue. Lifelong daily levothyroxine is required.

Is levothyroxine the same drug used in humans? Yes, the drug is the same, but dosing in dogs is quite different from human dosing. Human thyroid preparations can be used in dogs under veterinary supervision, but branded veterinary levothyroxine products ensure consistent dosing. Don't use human preparations without veterinary guidance.

My dog is on levothyroxine and seems much better - can we stop the medication? No. The improvement is because the medication is replacing the hormone the thyroid isn't producing. Stopping medication will result in a return of symptoms within weeks as T4 levels fall. The condition doesn't improve with treatment - the treatment replaces a function the body can no longer perform.

Are there breeds particularly at risk? Medium to large breeds are disproportionately affected - Dobermanns, Golden Retrievers, Cocker Spaniels, Irish Setters, Labrador Retrievers, and Boxers are among the more commonly diagnosed breeds. The condition is rare in cats.