Before I started taking T3 my thigh muscles hurt when kneeing down and riding a horse. Since being on T3 I have started riding again since I now have my head back on my body and am just strong enough to get the saddle on a pony.
But my thigh muscles still hurt whilst riding but are fine re kneeing. Have any riders on here had any experience of aching leg muscles and what cured them?
Many thanks
Oooh I know what you mean! I haven't ridden for years but I remember when taking it up again in my 30's the muscle soreness was pretty bad until you get the leg, core and back strength back again. The John Wayne walk was in full effect 
I think being hypothyroid also makes us prone to rather stiff, sore muscles.
I enjoy lifting and do a lot of barbell squats and it has taken time to get flexibility in my hips etc. This is especially so if one has been a bit sedentary and sitting a lot. I used to be so stiff and could just about do a half squat leaning into a Swiss ball against a wall but after time and patience can now comfortably get my hip joint below the parallel of my knee joint (admittedly I find this easier with the aid of special squatting shoes and knee sleeves!).
Google/ YouTube for leg adductor muscle stretches and exercises to strengthen and gently stretch the inner thigh muscles, such as side lunges. Also exercises for hip flexor and psoas muscle stretches. Feels a bit rank at first but gets easier if you do a stretch a day when you have a spare minute or two in the privacy of your bedroom or living room, no fancy equipment needed.
Good luck and enjoy the riding! 
I'm not sure that what my wife and I call "horse muscles" is in anyway caused by thyroid levels. More likely that muscles are just being stretched and used in ways that they haven't for a while. For us, "horse muscles" only got better from regular riding or from stopping riding. Occasional riding brings it back. Riding wider horses brings it back more.
Of course, having said that, anything that affects muscles in terms of aches, pains, recovery time etc., could make the "horse muscles" worse. So thyroid levels, vitamin levels etc., could make it worse, but the underlying cause is that riding horses is not what we evolved to do. The human frame evolved for walking and running, not sitting, and definitely not sitting astride a horse. We just choose to do that despite the aches that result
Oh yes! I have a Highland pony who is about as broad as he is long and it was agony when I started riding him after a gap of 2 years. Regular riding and some exercises to improve core, leg and back fitness will definitely help. I don’t know if my stiffness and pain was thyroid related or more to do with advancing age but it does get better. Hang on in there.
If you think about when you first learned to ride, you'll remember that you were sore for days. It'll be like that all over again, plus being hypo makes your muscles weak. Make sure you aren't gripping too much instead of using gravity and balance to keep your body directly over the horse's centre of gravity
I'd like to thank you all for your sage advice, your experiences and support.
I think one of the troubles is that I have never had any aching muscles from riding, not when I first started, nor if I went for an active long ride after a long gap nor when we rode long distances of 500 miles or so, hence I am now knocked sideways!! But I will persevere.....
And whether it is due to being hypo, or Parkinson's or age we are none the wiser.
Sore muscles
Fast twitching muscles on T3?
Aching joints with Hashimoto's
Does T3 cause weak muscles and ligaments?
Cramp and hard muscles 
