What are typical signs of hypothyroidism in older dogs, and how is it diagnosed?

• A. Lethargy, weight gain, heat-seeking, symmetric hair loss; diagnosed by low total T4 and free T4 with supportive tests; TSH stimulation may confirm
• B. Hyperactivity and weight loss
• C. Polyuria and polydipsia only
• D. Elevated T4 with tachycardia

Answer: (A)

In older dogs, hypothyroidism typically causes a slowing of metabolism, leading to lethargy, weight gain with normal or increased appetite, a preference for warm environments, and a dull, symmetric hair coat with thinning or loss on the trunk. Because these signs overlap with many other conditions, diagnosis uses thyroid testing alongside the clinical picture. The common approach is to start with measuring thyroid hormones: total T4 is often low, and assessing free T4 helps because it reflects the active hormone more reliably and is less influenced by binding proteins or concurrent illness. If low T4 is found but illness could be suppressing thyroid function, a TSH stimulation test can help confirm by showing a blunted or absent T4 rise after TSH administration. So, the combination of compatible signs with low total T4 and low free T4, supported by a stimulation test if needed, supports hypothyroidism. Signs like hyperactivity with weight loss don’t fit, polyuria and polydipsia alone aren’t typical, and an elevated T4 with tachycardia would point toward hyperthyroidism rather than hypothyroidism.