The management of FHT has been covered extensively in other publications, notably the 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism.² Much of the information in the 2016 AAFP guidelines remains applicable today. This discussion of FHT will serve to highlight certain key points and present recent findings related to the disease.

FHT is an extremely common endocrinopathy in cats, resulting from excessive circulating levels of T4 and T3. These excess hormone levels are most often caused by benign adenomatous hyperplasia of one or both thyroid glands and create a state of increased metabolism. Thyroid carcinoma is a rare FHT etiology, present in fewer than 3% of cats at the time of initial diagnosis.⁶⁷ Although FHT is typically diagnosed in cats older than 10 yr, elevated T4 levels in younger cats may be found more often as annual blood screening becomes more commonplace. It is important to note that T4 levels naturally decrease with age. A senior or geriatric cat with high normal T4 values according to the reference laboratory may actually be hyperthyroid. Following trends on annual screenings should reveal T4 levels either static or dropping. Elevation over time with development of clinical signs warrants further confirmation testing, even if the T4 level is technically within the laboratory’s normal range.