Fido or Fluffy may be lethargic. Or hyperactive. Eating less and gaining weight, or eating more and losing weight. Feeling cold. Feeling hot. The thyroid gland, which sits against the windpipe, could be responsible. Dogs aged two and up are at risk for developing hypothyroidism, a condition resulting from not enough thyroid hormone. Middle-aged and older cats may develop hyperthyroidism, which happens when too much thyroid hormone is produced.