Fluid retention was one of my biggest bug bears when I was trying to prove I was hypo.I called it Belly Bloat. I looked like the marshmallow man from Ghostbusters!
You will find however many women get fluid retention when starting the menopause.
It is always a good idea to get your kidney function checked, as I did, and found I was a little on the not so good side, and had to take some diuretics to kick those little buggers into gear.
I tend to get retention prior to the start of my period but that is normal.
I have Hashimoto's too.
Your doctor may be happy with your levels, but are you? Doctors are happy if levels are in range, but we often don't find our symptoms disappearing until we are well up in the ranges (upper half).
Please post FT3, FT4 and TSH results along with reference ranges (these vary lab to lab and have to come from your own lab report).
Do you have any other hypo symptoms?
Water retention is one of those symptoms that can be from either thyroid or menopause. I had terrible ankle, etc. swelling when I was hypo. In fact, after getting my meds adjusted correctly, I dropped 25 lbs without trying...probably mostly water, and that's a LOT of water!