The thyroid gland consists of two lobes and is located in the neck. This gland is responsible for the production of two hormones, T3 and T4, both of which contain iodine. These hormones affect many crucial processes in the body. Generally speaking thyroid hormones are there to regulate metabolic rate, which is simply the speed at which body processes function or “run”. If there aren’t enough of these hormones, the body will become sluggish. If there are too many, the processes will run too quickly.
When the thyroid hormone secretes it is regulated by a chain reaction of events that starts in the hypothalamus. The first step of the chain is for the hypothalamus to act on the pituitary gland by releasing a thyrotopin-releasing hormone. That hormone acts stimulates the pituitary gland which secretes thyroid-stimulating hormone. That hormone causes the thyroid gland to secrete T3 and T4 and the chain of events is complete.
A variety of cellular processes are influenced by thyroid hormones. Some reactions from thyroid hormones take only a few minutes or hours to occur, but others take hours or longer. If thyroid hormones are being released at a normal volume and consistency, they will work alongside other hormones, such as insulin and growth hormone, to build tissues in the body. But if there is an excess of thyroid hormones in the body they can actually begin to break down tissues and proteins.
Hypothyroidism In Dogs
Hypothyroidism is a condition wherein the levels of thyroid hormones in the body are decreased and a slower metabolic rate results. In almost every case of canine hypothyroidism – 95 percent – the cause of the hypothyroidism is the destruction of the actual thyroid gland. While pituitary gland tumors can also cause deficiencies in other pituitary hormones and also contribute to hypothyroidism, this is a rare case for canines.
If hypothyroidism is going to occur in a dog it will most likely occur between the ages of four and 10 years old. Mid- to large-size breeds are at the highest risk, and most toy and miniature breeds are not affected. Hypothyroidism is most common in Airedale Terriers, Doberman Pinschers, Cocker Spaniels, Golden Retrievers, Irish Setters, Miniature Schnauzers, and Dachshunds. Both genders are affected indiscriminately.
There are multiple symptoms of hypothyroidism due to the fact that it affects the function of virtually every organ system in the body. Most commonly dogs present with symptoms such as unwillingness or inability to be active, weight gain without an obvious increase in appetite, and lethargy. Sometimes dogs with this condition will have a hard time staying warm and seek out sources of heat. Dry skin, excessive shedding, hair thinning or loss, delayed hair re-growth and other skin signs are also common symptoms. In severe cases thickened skin can occur, especially around the face and forehead, which may make the dog look puffy. Skin folds above the eyes can also occur due to thickened skin.
If a dog hasn’t been neutered hypothyroidism can cause many different disturbances of the reproductive system. Some females have very irregular heat cycles or don’t have any at all, and most are infertile. If they do give birth, the chances of survival for the litter can sometimes be poor. Low sperm count, infertility, lack of libido, and small testicles are all reproductive symptoms of hypothyroidism in males.
Thyroid hormones are critical for proper growth and development of the skeleton and central nervous system during the fetal period and within the first few months of a dog’s life. If a dog is born with hypothyroidism, or if the condition develops early in life, diminished mental capacity and dwarfism are often symptoms. Depending on the cause of the hypothyroidism it may also be possible to detect an enlarged thyroid gland.
It is important to closely evaluate the symptoms of hypothyroidism and conduct a variety of lab tests when diagnosing hypothyroidism to ensure the diagnosis is accurate. Low thyroid hormone serum concentrations, particularly of T4, that don’t respond to thyroid-stimulating hormone in the lab likely indicate hypothyroidism.
In dogs with hypothyroidism treatment usually involves replacing a missing thyroid hormone or increasing the thyroid hormone. Thyroxine, or T4, is a hormone replacement for thyroid used most often in canines. Monitoring the improvement in symptoms in the dog can help to measure the success of the treatment. In most cases the treatment must be administered for four to eight weeks before body weight and coat changes can be seen and evaluated. Monitoring serum thyroid concentrations is also important to see whether or not the dosage of the treatment must be altered. After the veterinarian finds the most appropriate dose, the dog should have his or her hormone levels checked once or twice a year, and lifelong treatment is usually required.
Hyperthyroidism In Dogs
Hyperthyroidism is just the opposite of hypothyroidism and exists when there is too much of the thyroid hormones T3 and T4 in the body. Weight loss, increased heart rate, and increased appetite are all symptoms of this condition, which increases the metabolic rate in the body. However, hyperthyroidism is seen much more frequently in felines than canines.