This section is on the thyroid gland and things that can do wrong with it. If you have any questions about the information presented below please feel free to email me.
The primary function of the thyroid gland is the secretion of a hormone called thyroxine. Thyroxine is responsible for controlling metabolism, ie, its presence increases the metabolic rate. In so doing, it also increases the amount of food that is needed, the amount of body heat produced, and the level of cellular activity of all cells in the body. The cells that produce thyroxine take up iodine and attach it to a proteinaceous structure that is then modified to create the hormone. These cells form a circle around a storage space (follicle) into which they send the hormone for storage and are, thus, called follicular cells. When the body signals that it needs higher metabolism, the cells then resorb the hormone from storage in the follicle and secrete it into the bloodstream. A second type of cell known as parafollicular cell resides in the thyroid gland between the follicles and follicular cells. Parafollicular cells secrete a hormone called calcitonin that in conjuction with the hormone from the parathyroid glands regulates calcium levels.
The thyroid gland lies on the trachea ("windpipe") directly behind and in some species on the larynx ("voice box"). It is an oblong, rounded gland that is divided into two lobes in most of the domestic species. These lobes are connected by a thin strip of thyroid tissue or a thin strip of fibrous connective tissue called an isthmus. In contact with the thyroid gland are several (usually four) small glands known as the parathyroid glands. These glands secrete a hormone (aka parathyroid hormone or PTH) that with calcitonin regulates calcium balance in the body.
Hyperthyroidism is the disease condition in which the thyroid gland produces too much thyroxine. One cause of this can be the overstimulation of the thyroid gland by an overproduction of thyroid-stimulating hormone from the pituitary gland. Another cause is increased replication of follicular cells that may or may not be cancerous (neoplasia if cancerous, hyperplasia if not). The more cells the more total thyroxine that is produced. Still another scenario is the breakdown of the feedback mechanism that tells the thyroid gland that there is enough thyroxine produced and to stop producing it. Whatever the cause the end result is too much thyroxine. These abnormally high levels of thyroxine cause the metabolic rate of the animal to increase, and thus, the signs of hyperthyroidism are the result of overactive cells. The signs include weight loss, increased appetite with concurrent weight loss, hyperactivity, hyperexcitability, nervousness, hyperthermia (fever), panting, and palpable thyroid gland. This syndrome is most common in middle aged to older cats although it can be seen in dogs. Diagnosis can be made from the presence of clinical signs and the results of tests that measure thyroxine levels.
There are a variety of treatments that can be used to treat hyperthyroidism. Drugs exist that specifically target the follicular cells of the thyroid gland preventing thyroxine production, secretion, and / or conversion to the proper form. A surgical alternative, which is of greatest efficacy in benign, well-demarcated cancers, is to remove the part of the thyroid gland that is replicating out of control and, therefore, responsible for the oversecretion of thyroxine. More malignant cancers or ones that are diffusely spread throughout the thyroid gland are harder to treat by surgical removal. Great caution must be used in surgically treating thyroid problems in order to not damage the parathyroid glands. If these glands are affected, calcium balance will be lost and all kinds of complications will result. Another alternative that is very efficacious and has remakable few side effects is the injection of radioactive iodine into the animal. Thyroid cells are the only cells in the body that uptake iodine at appreciable quantities. Thus, the radioactivity of the iodine specifically targets the thyroid cells. The major drawback to this treatment is that the animal must be kept in isolation in a radiocative-proof area so that other animals and people are not exposed to the radioactivity. Suprisingly, the animals treated this way (almost exclusively cats) do extremely well and experience few side effects. There are several medications that are effective in some cases.
Hypothyroidism is the disease condition in which the thyroid gland produces too little thyroxine. Like hyperthyroidism there are several causes of this. The pitituary can produce too little thyroid-stimulating hormone, the follicular cells can be damaged by neck trauma, a nearby cancer can crowd the follicular cells causing cellular damage, the body can produce antibodies that cause the immune system to attack the follicular cells, etc. Regardless of the cause, the result is decreased or stopped production of thyroxine. The metabolic rate is, therefore, depressed. All the signs associated with this condition stem from the metabolic depression and decrease in cellular activity. These include weight gain with or without an appetite depression, lethargy, weakness, cold limbs and heat-seeking (decreased metabolic rate = decreased body heat), dulled responses, decreased heart and respiratory rates, recurrent infections (due to decreased activity of the immune system cells), diminished reproductive capacity (in females failure to conceive or even come into heat, in males decreased libido and sperm count), and the VERY common sign of bilateral (on both sides), trunkal (on the body), non-itchy alopecia (hair loss). This condition is most often seen in large breed, middle aged dogs. Diagnosis can be made from the presence of clinical signs and the results of tests that measure thyroxine levels.
Hypothyroidism is treated by supplementing the animal with thyroxine. This is a relatively uncomplicated treatment once doseage is established in otherwise healthy dogs. If, however, the dog has other endocrine or metabolic conditions, it becomes more complex and difficult to properly supplement thyroxine. In either case, the owner must constantly be on the watch for signs that the dose is too high (aka hyperthyroidism signs) or too low (return of hypothyroid signs).
Much debate rages in the veterinary community concerning this topic. To my knowledge no research exists to confirm or deny the link. There is, however, some evidence that suggests that a link does exist. Certainly, in a dog that suddenly develops aggression or other behavior problems, checking thyroid levels would not hurt and may help. An article concerning this topic is well worth reading.
I have researched and answered these questions to the best of my ability. But I am human and make mistakes. If there is any part of my answer that you feel is incorrect, please let me know. The last thing I want is to disseminate false or incorrect information.