I had low ferritin and vit D. I took bluebonnet chelated iron 27 mg, twice a day, with no constipation problems. I would take your iron 4 hrs after the cytomel and then again with dinner or before bed. I took for 2 months and brought my levels up. Now I just take one a day. I'm pretty sure you can take the vit D whenever. I take mine at night.
I know Calcuim binds with thyroid. I'm not sure if Vit D or iron is any problem with thyroid.
I take thyroid meds 2x/day (early morning and around noon), so I take things like iron in the late afternoon or evening. I take vitamin D with any meal I like.
I thought you had to wait 4 hours to take iron and calcium supplements.
Put thyroid at bedside with a bottler of water and take first thing. If on an empty stomach, you only need 30 minutes before you can eat anything, including the iron brand you like. Iron and vitamin D can be taken simultaneously. No worries.