My 3 year old neutered collie is losing her hair underneath. It is receding equally and the loss is speeding up.  She is losing about 2cm a day so in 2 weeks time it will meet across her back. Has anyone else experienced this with their dog?  I have a vets appointment tomorrow.  As she probably won't have a clue I'd like to have a few suggestions to put to her. Thank you in advance.

Thyroid problems can cause excessive hair loss.

^ This, and the pattern is typical of thyroid.  Ask for a full panel, and do not let the vet dismiss you (they can be a pain for this if it isn't a textbook case with a balding and very overweight dog).  You need to get Total T4, total T3, free T4, free T3 and TgAA antibodies.  TSH is no good for diagnosis, so also don't allow the vet to persuade you that it's the best test there is.

At an absolute minimum, if you can't get a full panel, go for free T4: this will tell you how much thyroxine is available to the body to convert to T3, not just how much is floating around in total (hence total T4).  But really you need the antibodies too, as they can skew result if they are flaring.

This could also be seasonal alopecia; not common in collies but not impossible.  When did it start?

I have copied everything you have written to show the vet.  It started a year ago after being groomed at a different place to where I usually go.  It was the inside back leg that looked like a gnat bite.  It then spread.   This information could be a red herring.  The vet has been prescribing drugs.  It was thought to be a metal allergy but now I'm not convinced.

That is interesting.  Has the vet done skin scrapes?  I wonder if the dog was nicked with something at the groomers and it triggered this, or it is indeed a red herring and something bit him around the same time and started it.  You say gnat bite - was it a raised red bump?  Tick bites often look like this after the tick has detached, and he could be having a lengthy reaction to it or possibly the tick was carrying something, etc etc.