Thyroid (hypothyroidism) in the dog

What is hypothyroidism?

The thyroid gland produces hormones that regulate metabolism. In hypothyroidism, the gland produces too few of these hormones. Over 95% of cases in dogs involve primary hypothyroidism: the gland itself is affected, either by autoimmune inflammation (lymphocytic thyroiditis) or by idiopathic glandular atrophy where glandular tissue is gradually replaced by adipose tissue [2].

Recognition: broad and non-specific symptom pattern

The clinical picture varies greatly between dogs and resembles many other conditions. Typical complaints are [1][2]:

• Lethargy and reduced desire for activity
• Weight gain without increased food intake
• Symmetrical hair loss that does not itch, often on the trunk and tail
• Dry, flaky skin
• Cold sensitivity
• Mental slowness or changed behaviour

Less common: neurological complaints, swollen face or increased susceptibility to skin infections.

Diagnosis: more than one blood value

A correct diagnosis requires the combination of appropriate clinical symptoms AND laboratory confirmation [2][3]. Relying solely on a low total T4 (TT4) is insufficient: any serious illness — also outside the thyroid gland — can temporarily lower TT4 (the so-called euthyroid sick syndrome). A complete thyroid panel includes at least TT4, free T4 via equilibrium dialysis and TSH.

The Merck Veterinary Manual describes hypothyroidism as 'one of the most overdiagnosed diseases in dogs' [2]. An incorrect diagnosis leads to unnecessary lifelong medication use and misses the actual cause of the complaints.

Proven treatment: levothyroxine

Levothyroxine (synthetic thyroid hormone, daily oral administration) is the only scientifically supported therapy and is recommended as such in both the 2023 AAHA guideline [3] and the Merck Veterinary Manual [2]. The treatment is:

• Lifelong — the gland does not recover spontaneously
• Effective: lethargy and behavioural changes often improve within 2–4 weeks [1]
• Hair growth usually recovers after 2–3 months [2]
• Regular monitoring of blood values is necessary to adjust the dosage

Natural and supplementary options: no evidence

There is no controlled scientific research that has proven any of the following interventions as an alternative or supplement in dogs with hypothyroidism [2][3]:

The lack of evidence does not automatically mean that these agents are harmful, but in a hormonal condition that requires lifelong medicinal correction, the risk of delaying or replacing levothyroxine is real and unacceptable.

When to see the vet?

Always consult a veterinary surgeon for diagnostics if hypothyroidism is suspected. Administering supplements before a diagnosis has been established can influence blood values and complicate the diagnosis. Treatment with levothyroxine requires a prescription and regular follow-up.

Supplementary ≠ substitutive. Levothyroxine is not an optional choice in confirmed hypothyroidism — it is the only proven correction for the hormone deficiency.