The thyroid gland is located in the neck, producing 2 iodine-containing hormones, T3 and T4 that affect many processes in the body. 
The hormones regulate the speed at which everything runs in the body or in other words, the metabolic rate of the animal. Too much of the hormone causes them to run too fast while too little causes the processes in the body to be rather sluggish.
The secretion of thyroid hormones starts in the hypothalamus, which acts on the pituitary gland by stimulating it to secrete thyroid-stimulating hormone. This, in turn, acts on the thyroid gland by stimulating it to secrete T3 and T4 hormones. Thyroid hormones affect many different cellular processes within minutes to many hours, working along side other hormones like insulin and growth hormone to build tissues. If they are secreted in excess, they can contribute to the breakdown of these tissues and protein.
HYPOTHYROIDISM
Hypothyroidism involves decreased levels of the thyroid hormones that result in a slower metabolic rate. Over 95% of cases of hypothyroidism in dogs is caused by the destruction of the thyroid gland itself. Another condition includes a tumor on the pituitary gland that causes deficiencies of these hormones as well. Hypothyroidism is quite common in dogs aged 4-10 years old, affecting mostly mid to large size breeds in both males and females. The condition is quite rare in miniature or toy breeds. The breeds most commonly affected include:
- Airedale Terrier
- Cocker Spaniel
- Dachshund
- Doberman Pinscher
- Golden Retriever
- Irish Setter
A deficiency of thyroid hormone affects all the organ systems, however the signs and symptoms do vary. Most of them are directly related to metabolism. They include:
- Lethargic behavior
- Lack of interest to exercise or run
- Weight gain without an increase in appetite
- Trouble keeping warm and constantly search for someplace warm or a heat source
- Dryness of the skin
- Excessive shedding
- Hair loss or thinning
- Delayed regrowth of hair
- Puffy appearance
- Slight drooping of the upper eyelids
- Irregular heat cycles in females
- Poor litter survival
- Lack of libido in males
- Small testicles
- Low sperm count
- Infertility in both males and females
Thyroid hormones are critical for the first few months of life, particularly for growth and development of the central nervous system and skeleton. Those born with a thyroid deficiency develop it early in life and often shown impaired mental development and dwarfism.
DIAGNOSING AND TREATMENT
Diagnosing hypothyroidism requires a thorough evaluation of their signs and symptoms, laboratory tests including demonstration of low serum concentrations of thyroid hormones, especially the T4 hormone, that do not respond well to administration of thyroid-stimulating hormone.
Treatment involves replacing or increasing the missing thyroid hormone, which is usually the T4 hormone. Typically, the signs and symptoms start to improve and usually take anywhere from 4 to 8 weeks to see noticeable changes. Sometimes the dosage of thyroid hormone needs adjusting, however once the dosage is stabilized, it only needs to be checked once or twice a year.
HYPERTHYROIDISM
An excess of the T3 and T4 thyroid hormones is hyperthyroidism. Signs and symptoms indicate an increased metabolic rate and include:
- Increased appetite and thirst
- Weight loss
- Vomiting and diarrhea
- Increased fecal volume
- Increased heart rate
- Shortness of breath
It is much more common in cats than dogs and is usually treated with radioactive iodine or an anti-thyroid drug. Sometimes surgical removal of the thyroid gland is necessary. Radioactive iodine is safe and effective and basically destroys the overactive thyroid tissue without affecting other tissues.
Reviewed and approved by Dr. David L. Roberts, DVM
Photo: Courtesy of cat-observer via Flickr (CC by 2.0)
