Thanks for the answer to my question.
Typically, I dose thyroid replacement drugs twice a day for my patients since this is the dosage regimen recommended by Plumb's Veterinary Drug Handbook and I've had great success in controlling my canine Hypothyroid patients with this schedule.
And, the amount of drug that I'd start with would be 3 mcgs twice daily for a dog this size. I always recommend a T4 recheck in 4-6 weeks to determine if this is the appropriate amount of drug for her and change my dosage (or not) accordingly.
However, having said that, while I rarely dose once daily there are some patients who will do just fine on this dosing especially if they are only mildly hypothyroid. But, I'd still want to check the T4 level and monitor for recurrence of clinical signs (such as weight gain, lethargy, skin issues, etc) if they are only receiving the drug once a day.
Your brother's decision to medicate once daily in the absence of a sound reason to do so is not the way I'd approach this patient if she were mine but, unfortunately, I can't unequivocally state that it's an incorrect dose.
If you can't convince him to treat her twice daily, then at least run another blood test 4-6 hours after she's been given her pill and evaluate the results. Or if she starts to put on more weight, then once a day is probably not enough for her.
I hope this helps although I'm sorry that I can't confirm that your brother is wrong....although it doesn't give me any pleasure to say so! Deb