Categories: Feline Health, Feline Nutrition. Hyperthyroidism is a common disease that typically affects middle-aged and older cats. It is caused by an excess production of thyroid hormones, which are produced by the thyroid gland, located inside the cat’s neck.Thyroid hormones affect nearly all organs, which is why thyroid disease can sometimes cause secondary problems such as hypertension, heart and kidney disease.ditor’s note: Soy is a common source of flavanoids, and is also a common ingredient in lesser quality pet foods. Soy protein is cheaper than meat protein, which is why manufacturers often use it to boost the protein content in pet foods. Unfortunately, cats lack the enzyme to properly metabolize plant-based proteins.