Welcome. You mention achy joints so perhaps your movements are restricted. I would suggest taking things gently at first. Short walks be they in your home from room to room with furniture to hold to or sit on or weather and ability permitting short ventures into the open air. Don't be tempted to do too much at first build up gently. Any movement above what you usually do can be classed as exercise. If you set aside a time of day with the intention of repeating your walk daily please remember two rest days are recommended in a week giving yourself time to acclimatise to the new activity.

When you know how much you can do comfortably you could branch out and join some exercise classes. As I am sure you know, what is good for one is not so hot for another. It very much depends on your level of stamina and what level of movement you have thyroid problems make things so much more taxing. It could be an idea to take some pain releaf before starting exercise but I found it made things worse but that is another story. You could well find your muscles remember what you used to ask of them. I had not been on a bike for years 20 or more and the action came back quickly. I kept this up for several year but since then I have not been on a bike for another 10 years.

All the best, steady as you go.

I second the suggestion to try swimming or aqua-batics class. The water will make it less work on your joints. That depends on if you have access to a swimming pool. If not, short walks are a nice start, as well.

I just started walking again. Also, how about yoga? You may find it helps. There is a website I really like called doyogawithme.com and it's free. There are tons of class videos from 10 minutes on up, and at different levels too.