Most of the online ones do not get their medications from the companies that make them. This may be true for all thyroid medications, but at least heartworm medications were that way and the companies that make them will not guarantee them ( many times they are from other countries and you do not know if they were kept at the right temperature for shipping, etc. I know one person that was going to buy, but the picture they had with the price was of a box that the company had not made for about two years and it was from one of the reputable online pharmacies. Makes you wonder if that is what they were still mailing out.
For thyroid medication you can check with local pharmacies, but usually the dog dose is different than human dose. I have a hypothyroid dog and she has been on medication for 5 years for it. The first script is usually a small one so that the blood levels can be retested to make sure they do not need to change the dosage. Once your dog is going to be on the same dosage ask about a larger quantity depending on what you usually get at a time. There is usually a pharmacy fee and then the cost of the pills, so a larger quantity means fewer pharmacy fees. . Soloxine is slightly more expensive if that is the thyroid mediation your beagle is on.