I have been struggling to get the thyroxine dosage correct for my 9-10 year old English Setter female. We adopted her 2 months ago after a lifetime in a shelter. Her blood tests showed that she has hypothyroidism (5.55 nanomoles/L on the thyroid test, where 13-53 is the normal range). After a couple of weeks on Forthyron 400 mcg (3/4 tab in the morning & 1/2 in the evening), she was responding positively but seemed to be panting excessively, and the next blood test showed 6.84 nanomoles/L; the vet told us to increase to 1 tablet in the morning and 1 at night. After a few days at the higher dose (which we were told is the min. recommended amount for her weight), she developed diarrhea, which did not subside despite charcoal and anti-diarrhea medicines, and then she developed an intestinal infection (mucous diarrhea). This was over a couple of weeks, and she lost a lot of weight. Treatment with Flagyl, as well as stopping the thyroxine, completely fixed the diarrhea problem. However, after a week or so without the thyroxine medicine, her skin became extremely flaky and itchy, her eyes started to get irritations and mucous again, she was lethargic and retaining water around her belly. Four days ago, we started with 200 mcg levothyroxin per day, and she immediately was revived and seemed happier. I still had the feeling she was a little excited, so I reduced her dose today to 100 mcg. A few hours after her tablet, the mucous and completely liquid diarrhea returned. I am trying to find out how to modulate the dosage. On one hand, the vets (and the Forthyron leaflet) tell me that 2 tablets per day (ie 10 mcg per kg of body weight, and she was 35 kg) each 12 hours is correct. That would make 800 mcg of levothyroxine per day. One vet informed me I could use human levothyroxine, as it is the same medicine. In the human leaflet, the total recommended dosage is 12.5-50 mcg per day total for a child, and up to 200 mcg for an adult. While I understand that the thyroid in humans and dogs is certainly different, it seems like a massive difference in dosage. Dog’s dosage: http://www.vmd.defra.gov.uk/productinformationdatabase/spc_documents/spc_252335.doc Human’s dosage: http://www.ies.org.il/PL_Levothyroxine_EN_2010Sep.pdf My understanding is that instead of following the first vet's advice, I should slowly ramp up the dosage, starting at a fraction of the daily dosage instead of the full dose. Does anyone have experience with finding the correct dosage, especially when levothyroxine is definitely causing diarrhea? Thanks for any advice.