Thyroid hormones: the hyperthyroid cat

Question 1

Thyroid gland?

Answer 1

-The thyroid gland lies on the ventral/lateral surface of the trachea, caudal to the larynx (it may overlap the larynx).
-In the dog and cat, the thyroid is separated into right and left masses, occasionally connected by an isthmus.
-In the horse, paired lobes are connected by a small isthmus
-In the cow, the lobes are connected by a wide isthmus.
-In the pig, the lobe is triangular; wide rostrally and pointing caudally.
-The thyroid gland functions for: Produce hormones promoting growth and metabolism. Store iodine.
-The thyroid gland secretes three hormones:
1.)Triiodothyronine (T3)
2.)Tetraiodothyronine or thyroxine (T4)
3.)Calcitonin
-Thyroid: stimulating hormone (TSH) from the anterior pituitary stimulates the synthesis and secretion of thyroid hormones T3 and T4.
-TSH does not stimulate the secretion of calcitonin. Calcitonin is secreted in response to high blood calcium levels.
-The thyroid gland contains numerous follicles (also called acini) whose walls are composed of simple cuboidal epithelial cells.
-These thyroid follicles contain a gelatinous material, colloid, composed of the protein thyroglobulin.
-Thyroglobulin is a scaffold protein for the T3/T4 synthesis, secreted by the epithelial cells lining the follicles and located in the colloid.
-Thyroglobulin holds and stores inactivate thyroid hormones and iodine in the colloid.

Question 2

Thyroid hormones-synthesis?

Answer 2

-The thyroid hormones T3 and T4 are formed in the colloid through linkage of two iodinated molecules onto the amino acid tyrosine.
-Thyroxine (T4) contains four iodine atoms.
-Triiodothyronine (T3) contains three iodine atoms.
-T4 is the main product of the thyroid gland, with a concentration 50-60 times greater than T3.
-T4 and T3 have the same functions, but triiodothyronine (T3) is 5 times more potent than thyroxine (T4).
-The thyroid glands also produce reverse triiodothyronine (rT3), which is biologically inactive.

Question 3

Thyroid hormones and iodine?

Answer 3

-To produce the thyroid hormones, the thyroid gland must be supplied with iodine.
-Iodine in food is absorbed by the small intestine and then circulates in the blood as the ionised form, iodide.

Question 4

Thyroidn hormones- secretion and transport?

Answer 4

-TSH stimulates the synthesis and secretion of T3 and T4.
-The thyroid hormones are lipophilic; therefore, mostly transported in the blood by binding to plasma proteins:
1.) Thyroxine-binding globulin (TGB) (70%).
2.) Albumin.
3.) Prealbumin
-Only the free unbound fraction in plasma can affect target cells.
-Thus, the thyroid hormones bound to TBG are a ‘backup storage’
-Changes in the physiological situation may affect the equilibrium between free and bound thyroid hormones.

Question 5

Thyroid hormones-metabolism?

Answer 5

• T4 (Thyroxine) is the main product of the thyroid gland and is deiodinated to the more potent T3 in the liver and kidneys
  -This deiodination provides more than 80% of the T3 in plasma.
  -Deiodination of T4 can also become the inactive form rT3.
  -The activation of the deiodinating enzymes is regulated by the body’s need for T3.

Question 6

Thyroid hormones-mechanism of action?

Answer 6

-Thyroid hormones are lipophilic and diffuse through cell membranes.
- In the cells, they bind to nuclear receptors and activate the transcription of certain genes.
-Functions of the thyroid hormones:
1.) Stimulate carbohydrate metabolism
2.) Stimulate fat metabolism
3.) Increase basal metabolic rate
4.) Decrease body weight
5.) Increase heart rate
6.) Increase respiratory rate
7.) Excite central nervous system- nervousness.

Question 7

Thyroid hormones-regulation?

Answer 7

-Thyrotropin-releasing hormone (TRH) is secreted from the hypothalamus.
-Thyrotropin-releasing hormone stimulates the anterior pituitary gland to release thyroid-stimulating hormone (TSH).
-Thyroid-stimulating hormone stimulates the synthesis and secretion of T3 and T4.
-Excess T3 and T4 suppress the secretion of thyroid-stimulating hormone and thyrotropin-releasing hormone (negative feedback).
*Maintain the level of T3 and T4 in a narrow, normal (healthy) range.
-Thyroid-stimulating hormone (released from the anterior pituitary) upregulates:
1.) The synthesis and secretion of thyroid hormones.
2.) The number and size of the follicular cells
-If the concentration of thyroid-stimulating hormone is increased for a prolonged period, it causes an increase in the number of thyroid follicular cells (hyperplasia) and the volume of each cell (hypertrophy).
-This leads to an enlargement of the thyroid gland, which can be palpated as a goiter on clinical exam.

Question 8

Thyroid dysfunction-Hyperthyroidism?

Answer 8

-Increased production of thyroid hormones, because of:
1.) Excessive secretion of thyroid hormones.
2.) Dietary-associated hyperthyroidism
3.) Excessive secretion of thyrotropin-releasing hormone
4.) Excessive secretion of thyroid-stimulating hormone.
-Frequently seen in cats, most commonly due to benign tumours of hormone-producing cells in the thyroid gland.
-Clinical signs can include:
1.) Increase appetite
2.) Weight loss
3.) Overdrinking
4.) Hyperactive and nervous
5.) Heart rate increased.
6.) Respiratory rate increased.

Question 9

Thyroid Dysfunction- Hypothyroidism?

Answer 9

-Hypothyroidism is due to reduced production of thyroid hormones, due to:
1.) Primary (failure of the thyroid gland)
2.) Secondary (thyroid-stimulating hormone deficiency)
3.) Tertiary (thyrotropin-releasing hormone deficiency)
4.) Inadequate dietary intake of iodine.
-Hypothyroidism is a common endocrine disease of dogs, most commonly due to primary causes from autoimmune damage to the follicular cells.
-Clinical signs can include:
1.) Reduced heat production
2.)The animal is sluggish and slow
3.) Weight gain
4.) Hair loss or dull hair
5.) Reduced heart rate
6.) Reduced intestinal movement

Question 10

The parathyroid glands?

Answer 10

-The parathyroid glands are located close to or embedded within the thyroid gland.
- There are usually four parathyroid glands (two on each side).
-In the dog, cat, and small ruminants, the parathyroid glands generally become recessed or embedded within the thyroid gland.
-The parathyroid glands are a pale colour compared to the thyroid gland.

Question 11

The parathyroid hormone (PTH)?

Answer 11

-Parathyroid hormone regulates calcium metabolism:
1.) In the gut: increase absorption
2.) In bones: release calcium to the blood (mobilisation of calcium from the skeleton).
3.) In the kidney: suppress calcium release in urine; activate vitamin D.
Increase the blood calcium level
-The production of parathyroid hormone is mainly regulated by the calcium concentration in the plasma. (Low calcium level => Parathyroid secretion increases).