Thyroid Testing (T4 and others)

The thyroid gland is a small, butterfly-shaped, gland located in the neck along the surface of the trachea (windpipe). The thyroid gland produces hormones which regulate vital body functions including body weight, heart rate, muscle strength, cholesterol levels, skin health, and much more.

  • T4 (Thyroxine)This is often the first test in evaluation thyroid function. Increased T4 can indicate a condition called hyperthyroidism (over-production of thyroid hormone), which is common in cats. Decreased T4 can either indicate a condition called hypothyroidism (under-production of thyroid hormone), which is common in dogs, or can be an indication of illness somewhere else in the body (called euthyroid sickness). To differentiate between true hypothyroidism and euthyroid sickness, follow-up testing is required. Dogs and cats with thyroid disorders require life-long medication to manage these conditions.
  • “Thyroid Profile” (T4, FT4 and TSH): The thyroid profile evaluates T4 (described above) in addition to two other thyroid-related hormones to help differentiate true hypothyroidism from euthyroid sickness. This is important because dogs with true hypothyroidism require life-long medication to manage this condition, whereas dogs with euthyroid sickness do not.
    • T4 (Thyroxine): See above
    • FT4 (Free Thyroxine): FT4 is similar to T4, except that it is not protein-bound and can be a slightly more sensitive measure of hormone levels in cats.
    • TSH (Thyroid-Stimulating Hormone): TSH is released by the body to signal the thyroid gland to produce MORE thyroid hormone (T4). A dog with true hypothyroidism will have a low T4 and a high TSH, whereas a dog with euthyroid sickness will have a low T4 and a normal TSH.