The thyroid gland is made up of paired lobes on the ventrolateral surface of the proximal trachea. Felines may have accessory
thyroid tissue in the neck and thorax. The size of the thyroid gland varies with the size of the dog; with medium-sized dogs
having lobes approximately 5 cm. in length, 1.5 cm. in width, and 0.5 cm in thickness. In the average (4.5 kg) cat each lobe
is approximately 2 cm. in length, 0.3 cm. in width and 0.5 cm. thick. The thyroid gland is fed by the cranial and caudal thyroid
arteries, arising from the common carotid artery. The external parathyroid glands lie on the surface of the thyroid lobes,
while the internal parathyroids are embedded within the thyroid tissue. Histologically, the thyroid gland is comprised of
rounded follicles that are lined by epithelial cells. The epithelial cells produce colloid, a gel-like glycoprotein which
is stored in the follicles. Colloid contains thyroglobulin; the precursor for the synthesis of thyroid hormones.
Synthesis of thyroid hormones requires iodine which is absorbed from the small intestine, converted to iodide, bound to plasma
proteins and transported to the thyroid gland. Within the thyroid gland, iodide is oxidized and bound to thyroglobulin molecule
as tyrosine residues; which in turn result in the synthesis of triiodothyronine (T3) and thyroxine (T4). T3 and T4 are stored
extracellularly. In order for T3 and T4 to be released into the circulation, thyroglobulin must re-enter the follicular cell
and undergo proteolysis.
In the circulation,T3 and T4 are tightly bound to plasma proteins (~99%), thus <1% are the free hormones, capable of exerting
metabolic effects. Most of the hormone produced by the thyroid gland is T4. In the target tissues T4 is metabolized to T3
(metabolically active) or reverse T3 (metabolically inert).
T3 and T4 affect almost all of tissues in the body; increasing the cellular metabolic rate. They are vital for normal fetal
development, calorigenesis and carbohydrate, fat and protein metabolism. Other effects of thyroid hormones include: erythropoiesis,
formation and resorption of bone, increased heart rate and strength of contraction and increased oxygen consumption by the
Regulation of the thyroid gland revolves around the hypothalamus (TRF) and anterior pituitary gland (TSH). Increased plasma
levels of T3 and T4 will reduce the release of TRF and TSH in a negative feedback manner. Glucocorticoids, androgens and GH
(all in supraphysiologic concentrations) will lower the secretion of TSH.
This is one of the most common endocrinopathies in the dog and is the result of lowered levels of T3 and T4. Hypothyroidism
is most commonly seen in medium to large-sized middle-aged dogs (golden retriever, Labrador retriever, Doberman pinscher,
Irish setter, boxers, Great Danes and Airedale terriers). Smaller breeds that are commonly afflicted include: dachshounds,
miniature schnauzers and poodles. There does not seem to be a sex predisposition but neutered dogs seem to be at a greater
Most dogs develop hypothyroidism as a result of lymphocytic thyroiditis (an immune-mediated disorder) or idiopathic thyroid
atrophy. Secondary hypothyroidism due to pituitary disease and lack of TSH secretion is very rare. Excessive cortisolinemia
(either iatrogenic or naturally-occurring) can also result in reversible secondary hypothyroidism.
Hypothyroidism usually progresses gradually and its symptoms are often non-specific. Often the pet owner is unaware of a problem.
As the condition progresses lethargy, weight gain (in absence of an increase in food consumed), obesity, depression, are noticed
by the pet owner. Dermatologically, a bilateral symmetrical truncal alopecia with a secondary seborrhea, hyperpigmentation
and pyoderma may be seen. Occasionally, signs of neuromuscular disease may accompany the clinical picture ( peripheral vestibular
disease, facial paralysis, generalized neuromuscular weakness, laryngeal paralysis and megaesophagus). Hypothyroidism may
also have a profound effect upon the reproductive tracts of intact males and females (testicular atrophy, decreased sperm
production, decreased libido, infertility, silent estus, prolonged interestrus cycles etc.).