Normal Thyroid Levels in Cats
All mammals have a thyroid gland, a very important hormone-secreting gland that affects all parts of the body. Thyroid levels need to be maintained at a constant level on a daily basis in order to sustain bodily functions. As a cat ages, one of the most frequent diseases to strike is hyperthyroidism. This fairly common condition can be controlled by medication. Understanding how the thyroid gland functions in the cat is an important step to understanding your cat's overall health.
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Thyroid Gland
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The thyroid gland in cats is a bi-lobed structure located next to the trachea. In addition to the thyroid gland, many cats have extra thyroid tissue elsewhere in the chest cavity. This ectopic (out of place) tissue can function similarly to the tissue found in the thyroid gland, and can also develop the same problems as the thyroid gland.
Thyroid Hormones
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There are two main types of thyroid hormones: thyroxine (T4) and triiodothyronine (T3). In a cat's body, T4 can be converted to T3, which is the more active hormone. Normal T4 levels in a cat are between .8 to 4 ug/dL. When a cat's thyroid gland is working normally, the levels of T4 will remain fairly consistent, with just minor fluctuations throughout the day. The levels are adjusted via a negative feedback loop between the thyroid gland and the pituitary gland.
Regulation of Normal Levels
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Thyrotropin-releasing hormone (TRH) binds to cells in the pituitary gland to stimulate the secretion of the thyroid-stimulating hormone (TSH). TSH then binds to cells in the thyroid gland to stimulate the secretion of thyroid hormones. When enough hormones circulate in the blood, TRH stimulation on the pituitary ends. When the body senses that more hormones are needed, TRH will again stimulate the pituitary gland.
Function
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Normal levels of thyroid hormones in the cat keep the body's cells functioning smoothly. All body cells require the action of thyroid hormones for metabolism. When hormone levels fall to a critical low level, body metabolism slows. Conversely, if hormone levels increase greatly, body metabolism increases. Both of these conditions are undesirable and can lead to serious long-term health consequences.
Abnormal Levels
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If a cat does have a thyroid disorder, most frequently it is an excess of thyroid hormone production. Nodules will form on the thyroid tissue and elevate thyroid hormone levels. This is called hyperthyroidism, and will need to be treated. Drugs can be given to suppress the production of the thyroid hormones, or the nodules can be destroyed with radiation treatment. The thyroid gland can also fail to function normally, leading to hypothyroidism, though this is a rare condition in cats.
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