Hello, I'm Dr. Deb. I'll do my best to help you today.
I'm sorry for this concern for Mia.
As you may know, this drug can be given to dogs as well as humans to treat hypothyroid disease but the dose in dogs is much higher than it is for us. The normal dose for a dog this size would be about 70 micrograms so for her having eaten 100 mcg wouldn't constitute a toxic
or evenly terribly high dose for her.
Symptoms which might be seen (especially in cases of chronic overdose but much less likely in one time, acute ones) include vomiting, diarrhea, hyperactivity to lethargy, hypertension, increased heart and respiratory rate, and abnormal pupillary light reflexes. These signs might be seen about 1-9 hours after the pill was ingested.
But, to be honest, I very much doubt that she'll develop any symptoms at all from the ingestion of this much Synthyroid as a one time event.
I hope this helps to reassure you that most likely she'll be fine. Deb