Thanks for your question, my name isXXXXX and hopefully I can be of assistance.
The usual starting dose I use for hypothyroid dogs is .02mg/kg, however I sometimes adjust this if the dog is overweight. That being said, for a dog that is 46 lb., I would start them on a dose as listed below with instructions for a recheck T4 (thyroid hormone level) 4-6 hours after their pill in 1 month.
RX: Levothyroxine 0.4mg - 1 tab by mouth twice daily
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I wish you and your dog the best, XXXXX XXXXX feedback is much appreciated.