I thought I would give this website a go and see if anyone knows if Levethyroxine can cause a reaction when using Durphatt2800 toothpaste.
Thyroxine & Durphatt 2800 toothpaste - Thyroid UK
Many patients on levothyroxine choose fluoride free toothpaste
This Durphatt 2800 contains double the standard amount of Fluoride of normal toothpaste
In case anyone wishes to look it up, I think the product is Colgate Duraphat 2800ppm.
Disappointingly, I have been unable to find the full ingredients on Colgate's own site. But there is a version on the MHRA site:
Active substance: 0.619% w/w Sodium Fluoride. 1 g of toothpaste contains 2.8mg fluoride, corresponding to 2800 ppm fluoride. Fluoride 2800 ppm Toothpaste is a white coloured toothpaste. The tube contains 75 ml toothpaste
Other ingredients: Glycerol (E422), Purified Water, Sorbitol (E420), Silicas, Macrogol 600, Sodium Laurilsulfate, Carmellose Sodium (E467), Mint Flavour, Titanium Dioxide (E171 ), Sodium Saccharin (E954).
Some people are intolerant to glycerol and sorbitol - but usually in significant amounts and if swallowed.
I don't think it at all obvious why levothyroxine would cause a reaction to the toothpaste. However, looking at it the other way round, fluoride does impact the thyroid.