It was very encouraging. He's put on 600g ( over 1lb 4 oz) in the 10 days since he started on the thyroid medication, which is a pretty good sign that it is working.
Also his glucose levels are in the 'normal' range, which suggests that either he doesn't have diabetes, or that he does, but when he is eating normally and in better health generally it's stable.
We're going back in 2 weeks, when they will do further blood tests to check on his thyroid (by which time he will have been on the meds for a month) and see whether the dosage needs adjusting, but so far, things are looking much better than I feared, 2 weeks ago.
Thank gods for sardines!