Do I need to worry about my dog's back?

[fairgame84]

Over the past 6 weeks or so I've noticed a bit of a dip/dent in my dog's back.
It's not squishy and doesn't bother him.
He went to the vet earlier last month and I forgot to mention it but she didn't notice it.

I think I'm worried because 3 years ago he had surgery on that part of his back to remove a cyst caused by heart worm injections so I don't know if it's related.

He is cross breed.
8 years old.
Has hypothyroidism.
Weighs 36kg.
Well in himself.
Eating, drinking, peeing, pooing as normal.
Not affecting his mobility.

Photos attached.

Is it his coat or can you feel a dent in his frame? It does look unusual to me, but I could believe it was coat thinning given his hypothyroidism, or it could be thinner coat over his surgical scar.

If it’s not directly where his surgery was I would talk to the vet.

I can definitely feel a dent in his frame.
He coat is really thick at the moment and doesn't seem thinner over the dip.

It's more visible when he lays down.

Hmm could be a bit of muscle wastage. Despite being active, my dog with hypothyroidism is definitely not as muscular as he was.

I would make a nom-urgent vet appointment. It may be something simple, like general ageing, aggravated by his thyroid, but if it’s worrying you, best to check.