Indy was "old thyroid" when she was diagnosed. About 10 years old (mixed breed), thyroid levels decreasing. Kelp helped, but not enough, so I put her on the meds.
Max (GSD) was completely opposite. Young. Low-normal free T4, and low free T3, and elevated antibody levels (TGAA). The vet wouldn't treat because he said Max "didn't look hypothyroid". I had to change vets because of that. The vet we go to now, we were the first ones that she treated without being comletely out of range, and she was very pleased with his response. His coat changed, his personality change.
It's important to get that full panel with the Free T4 and the Free T3, and the TgAA, which is the best way to determine autoimmune thyroiditis.