Canine hypothyroidism: skin signs and diagnosis
7-year-old neutered male Labrador Retriever with weight gain, lethargy and heat-seeking behaviour. It has non-pruritic symmetrical truncal alopecia, a 'rat tail', thickened facial skin and two episodes of superficial pyoderma in the last year. It is not on any medication.
| Parameter | Result | Reference |
|---|---|---|
| Total T4 | 0.6 µg/dL | 1.0–4.0 |
| Canine TSH | 1.2 ng/mL | < 0.6 |
| Cholesterol | 420 mg/dL | 135–270 |
| Haematocrit | 34% | 37–55 |
What is the most appropriate interpretation?
chevron_right Show answer and explanation
A. Non-thyroidal illness (sick euthyroid) syndrome: repeat the tests in 6 months
Sick euthyroid syndrome requires a concurrent illness that this dog does not have, and TSH is usually normal. Delaying treatment is not justified.
B. Primary hypothyroidism: start levothyroxine and monitor post-pill T4
Correct. Low total T4 with high TSH in a dog with compatible signs is highly specific. Hypercholesterolaemia and mild non-regenerative anaemia support the diagnosis. Check T4 4–6 h post-pill after 4–6 weeks.
C. Hyperadrenocorticism: perform a low-dose dexamethasone suppression test
Hypercortisolism can lower T4, but it does not raise TSH and would cause polyuria/polydipsia, polyphagia and thin skin, not thickened skin and lethargic weight gain.
D. Inconclusive: free T4 by dialysis must be measured before treating
Free T4 by dialysis is useful when results are discordant, but here the combination of low total T4 + high TSH already confirms the diagnosis.
lightbulb Key takeaway
Typical skin signs: non-pruritic symmetrical alopecia over friction areas and tail, poor regrowth after clipping, seborrhoea, hyperpigmentation, facial myxoedema ('tragic expression') and recurrent pyoderma or otitis; pruritus only appears with secondary infection. Up to 25% of hypothyroid dogs have normal TSH, so a normal TSH does not rule it out.
Test yourself with 10 random questions
Dermatology · 100 questions, a different test every time
Related questions
Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.