Canine hypothyroidism: total T4, free T4 and TSH
7-year-old neutered male Golden Retriever with lethargy, weight gain without dietary change, tail alopecia and cold intolerance. Mild normocytic normochromic anaemia. No medication.
| Parameter | Result | Reference |
|---|---|---|
| Total T4 | 0.6 µg/dL | 1.0–4.0 |
| Free T4 (equilibrium dialysis) | 3 pmol/L | 8–40 |
| Canine TSH | 1.2 ng/mL | < 0.6 |
| Cholesterol | 420 mg/dL | 135–270 |
How do you interpret the thyroid panel?
chevron_right Show answer and explanation
A. Sick euthyroid syndrome; repeat once the cause has resolved
In sick euthyroid syndrome T4 falls, but TSH is usually normal and there is obvious systemic illness. This dog has classic thyroid signs and a high TSH.
B. Confirmed primary hypothyroidism
Correct. Low total and free T4 with high TSH, in a dog with compatible signs and no drugs or other illness, is a highly specific combination. Hypercholesterolaemia and mild anaemia support it.
C. A TSH stimulation test is needed before treating
The recombinant TSH stimulation test is useful in equivocal cases, but here the panel is already conclusive. It is unnecessary and expensive.
D. Antithyroglobulin antibodies must be measured to confirm it
Antibodies indicate lymphocytic thyroiditis but do not measure thyroid function: they can be positive in euthyroid dogs and negative in hypothyroid dogs with idiopathic atrophy.
lightbulb Key takeaway
Low T4 with high TSH confirms hypothyroidism, but a normal TSH does not rule it out (25–35% of hypothyroid dogs have a normal TSH). Free T4 by dialysis is least affected by drugs and illness: use it in equivocal cases.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.