Joint supplement for a dog with hypothyroidism — Cocker Spaniel

Hypothyroidism and joint stiffness often appear together in Cocker Spaniels at this age — partly because low thyroid function causes muscle weakness and lethargy that looks like joint pain, and partly because the two genuinely do co-occur. Has the hypothyroidism been recently diagnosed, and has she been on levothyroxine long enough for the vet to say she's stable?

The energy improvement tells you the thyroid is being managed — the persistent stiffness suggests a genuine joint component on top of it, which is common. Hypothyroid dogs are also at slightly higher risk of myopathy, so I'd frame the supplementation as supporting both the joint and the surrounding muscle. For her specifically I'd recommend YuMOVE Senior — the green-lipped mussel anti-inflammatory effect is the primary value here, and it's a formulation with a straightforward safety profile alongside levothyroxine. Mention it at her next thyroid check so the vet can keep an eye on both in parallel.