Is that from your road bike or your mountain bike?
Me too!!!! from my mountain bike. Same pain you describe.
I had thumb tendonitis from riding my mountain bike with its "round" grips and straight handlebars. The pain was in the thumb joint close to my hand.
Mine was cured, YAY!!!!!, by changing my straight handlebars to ones with a slight curve and buying ergonomic handlebar grips with "cow horns."
Here is a photo of them... although I didn't buy them on eBay and this isn't meant to be an advertisement. compare.ebay.com/like/160619940565?var=lv&
(I also discovered that my knitting was really aggravating my thumb, although you probably don't knit.)
“The only way to keep your health is to eat what you don't want, drink what you don't like, and do what you'd rather not.”
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