I have a dog with a serious hypo-Thyroid.
I also own & live with his parents & 3 siblings; They are a rare breed dog - Xoloitzquintle, mine being a sub genus - Peruvian Incan Orchid dog.
Baba is the only one with a Thyroid problem. He was born large & at 4 months old displaying fear of people & cars but I thought it was bcz he had not been exposed to a city (Santa Barbara) before. Because I was dealing with 2 other dogs with terminal cancers, it wasn't until he was almost 2yro that he reached a point af huge weight & Autism that realized what was wrong.
We joked about his balooning weight but one day he could not recognize me 2ft away but acted like he 'knew' I was around but he was unable to recognize any thing moving.
Because I study & use supplements & the night before had been reading a research tract on a comparison of childhood Autism & senior Dementia & proteins that It dawned on me possibly what was happening; the Autism, not the Thyroid. I had Tyrosine powder in the house & immediatly gave him a dose. Within an hour he could recognize me with very limited response. I stepped up the dosage the next day & study.

So to shorten this a little.... I give him Thyroid Glandular supplements every day, Thyroid liquid Homeopathic also, & He knows to come & ask for them if I forget & that he feels better. He also loves fish oil capsules, vita E & A & B12, cares less for but gets MSM (speeds healing etc) & digestive enzymes. Once in a while I give him a DHEA if I am home to monitor his upped vitality & Iron. Also in his feed I vary mixing Alfalfa, Kelp etc but they all get that.
After I followed this program 'bout 6 months I decided to go for the Thyroid drugs from the Vet as his weight loss had plateaued at large. I explained to her what supplements I was using etc & she agreed it was a classic case & would run the blood panels. They came back normal & she charged me $150 to tell me my dog was fat!!
But the real is that the test showed that the supplements had raised his hormone levels to a NORMAL range!
I have had him on this natural program for 3+ yrs & he is great unless I skip 2 days in a row & he gets slightly low & sits at the vitamin cupboard to remind me....

So I write all this long post in case this info is useful to any one else. Our dogs don't have biases about vitamins or drugs; they work physiologically/biochemically or don't, so it acts as a blind if not dbl blind test. Glandulars are available in Health stores but not widely used by the public - Naturopaths, Chiropractors are schooled in them. I personally have used Adrenals for 20+ yrs for my overused, had too much fun & stress....

Tyrosine is an Amino Acid & tricky. The Thyroid uses a lot of it - the Kelp-Thyroid connection. It is the 2nd step in the Phenylalanine pathway which is a necessary AA for humans to ingest but enzyme tricky hence PKU which babies are tested for.
So for what it's worth...