Hi Lisa, yes I had a Great Dane with an underactive thyroid.
It was only because I persevered with the vet that a test was done to determine the thyroid activity, and it came back as at the lower end of what is regarded as "normal".
She was put on 16mg daily by two 8mg tablets (which at that dosage are only vet available), but they ran out at one stage and we were given a prescription to take to the Chemist and got it there, but it only comes in 1mg tabs for humans, so she was on 16 per day.
The Chemist did a double take on the script and asked to see the "patient", which I happily obliged him wit a visit from her, complete with prescription held in her mouth.
It made a vast difference to her overall wellbeing, previously lethargic she gained a new lease of life.