What is Hypothyroidism?
Hypothyroidism is an endocrine disorder that results from an “underactive” thyroid, a “butterfly” shaped gland located in the neck. The hypothalamus, a small region of brain located above the pituitary gland releases thyrotropin releasing hormone (TRH), which binds to the receptors on the thyroid initiating the release of the thyroid stimulating hormone (TSH). The thyroid stimulating hormone regulates the amount the iodine-containing hormones triiodothyronine (T3) and thyroxine (T4) secreted from the thyroid gland. The thyroid gland mainly secretes T4 which is converted to T3, a more potent form in the peripheral tissues. The majority of T4 and T3 is bound to plasma proteins. Free T4 and T3 is active and immediately available for uptake into cells. Hypothyroidism occurs when levels of T4 and T3 are decreased.
In order to maintain homeostasis, T4 exerts negative feedback on the hormones TRH and TSH stopping their release when they are not needed. These thyroid hormones play an important role in metabolism, regulating body temperature and heart rate.
Hypothyroidism is classified as primary or secondary depending on the origin of cause. Primary hypothyroidism is the most common type and is caused by a disease at the level of the thyroid gland, typically from either lymphocytic thyroiditis (immune-mediated) and idiopathic atrophy of the thyroid gland. Secondary hypothyroidism results from inadequate secretion of TRH from the hypothalamus or TSH from the pituitary gland.
Hypothyroidism is much more common in dogs than cats. It occurs most frequently in medium to large sized breeds between the ages of 4 and 10 years old. Certain breeds predisposed to hypothyroidism include: Golden Retrievers, Doberman Pinschers, Irish Setters, Miniature Schnauzers, Dachshunds, Airedale Terriers and Cocker Spaniels.
Clinical Symptoms Include…
- Weight gain minus increased caloric intake
- Intolerance to hot or cold temperatures (commonly will seek heat)
- Bilateral symmetrical alopecia (hair loss on both sides of the body)
- Brittle and dry hair
- Skin and ear infections
- Anemia (low red blood cell count)
- High cholesterol
- Reproductive issues, such as infertility
Thyroid Blood Panel
A thyroid panel is test used to evaluate thyroid functioning. It includes the concentration of T4, free T4 (active form) and the thyroid stimulating hormone (TSH). A concentration of T4 and free T4 below the normal reference range and TSH normal or above the normal reference range is indicative of primary hypothyroidism, however, there are non-thyroid diseases that can cause T4 levels to decrease. If all of these concentrations are within the normal reference range, hypothyroidism can be ruled out
There is no cure for hypothyroidism, but it can be managed with an oral medication called levothyroxine, a synthetic thyroid hormone. Brand names include Soloxine and ThyroTabs.