Thyroid glands
These lie in the midline on the ventral aspect of the first few rings of the trachea IEig 6.11. They are controlled by thyroid or thyrotrophic stimulating hormone (TSH) from the anterior pituitary gland, and secrete three hormones:
Thyroxin (or TJ and triiodothyronine (orTj - have a similar effect.
Tri-Iodolhyronine contains a high proportion of the trace element iodine - a lack of iodine in the diet can have a dramatic effect. Both hormones affect the uptake of oxygen by all the cells in the body and are essential for normal growth.Undersecretion - hypothyroidism - is more common in dogs. In young animals, hypothyroidism causes dwarfism (stunted growth). In older animals, the condition is known as myxoedema: the dog becomes fat and sluggish, alopecic. the skin feels cold and clammy and the heart rate slows - all due to a reduced metabolic rate.
Oversecretion - hyperthyroidism - is more common in old cats.The affected animal is thin, active, often aggressive.
has a good appetite and a fast heart rate - all due to a raised metabolic rate.
Calcitonin - lowers the levels of blood calcium bv decreasing the rate of bone resorption. When Ievelsof blood calcium are high. e.g. if a calcium rich diet is eaten, calcium is deposited in the bone and acts as a reservoir for later use (Eig. 6.2). Calcilonin has an opposite effect to parathormone but is of less importance.