I am sorry to hear that your pup Remi ingested another dog's 1mg thyroid pill today.
A usual dog dose for thyroid supplements is 0.022mg (22mcg) per kilogram (2.2 pounds per kilogram) of body weight twice daily. (There are 1,000 mcg per mg).
That means a 40-50 pound dog (18.2 to 22.72 kilogram dog) would normally take 0.4mgs to 0.5mgs.
And a 50 pound dog (22.72kgs) would normally take 0.5mgs or 500mcgs, so she took just about a double usual dose.
The good news is that a one time double dose of thyroid supplement pills thyroid pills will not be an issue in an otherwise healthy dog.
Even though she did get something she doesn't need a one time dose won't hurt your pup, if she is otherwise healthy with no history of heart disease, although it may make her a little hyper and elevate her heart rate slightly.
If you are going to see symptoms she should show some within the first 4-6 hours. The medication from a one time dose should clear from her body completely within 12-24 hours.
Honestly this one time dose should not be a problem for your pup, and the effects won't last more than 24 hours or so.
Please let me know if you have any further questions.