Thyroid gland Flashcards
What are the thyroid hormones?
T3 triiodothyronine
T4 thyroxine
What are the output % of T3 and T4?
T4 (thyroxine) - 90% of output
T3 (triiodothyronine) - 10% of output
Which is more biologically active: T3 or T4, and why?
T3 is far more active because of higher affinity to thyroid hormone receptors
What is the main precursor in all vertebrates to thyroid hormones?
Thyroglobulin
It’s produced from iodine in thyroid epithelium
What is the hypothalamic-pituitary-thyroid (HPT) axis?
- Neurons of the hypothalamus secrete TRH
- Anterior lobe synthesizes and releases TSH
- T4 and T3 are released in thyroid gland
- T4 and T3 have negative feedback loops both in thyrotrophs and in hypothalamus
How are T4 and T3 circulating?
More than 99% bound to proteins
Primary binder in dogs - Thyroid-binding globulin (cats don’t really have)
Primary binder in cats - Prealbumin
Hormones bound to proteins are not active, serve as reserve (equilibrium baby)
Factors that affect thyroid hormone concentrations
-Many diseases
-Pharmacological agents (i.e. glucocorticoids)
-Breed differences (i.e. small breeds have more TT4)
Metabolism of thyroid hormones
Deionation is most important thing!
-Liver and kidney deionates T4 into T3 (increases biological activity)
Stuff that affects deionation:
-tissue specific deiodinases
-fasting
-nonthyroidal illness
How quickly does thyroid hormone act?
There is a lag time of hours or days before full physiological effects take place
Physiological effects of thyroid hormones
Help maintain normal:
-Energy levels
-Weight
-Thermoregulation
-Heart rate
-Bowel movement
-Mood
4 types of hypothyroidism in young animals:
-Acquired juvenile hypothyroidism
-Thyroid dysgenesis
-Defective thyroid hormone synthesis
-Central (secondary) hypothyroidism
Prominent sign of juvenile hypothyroidism
Dwarfism (+ also all the “normal” signs)
Thyroid hormones are crucial for growth and development of all body tissues, particularly the skeleton.
What causes acquired juvenile hypothyroidism?
Iodine deficiency
i.e. diet with only meat without additives is deficient in iodine
In severe cases: lack of iodine –> TSH-induced thyroid hyperplasia, “goiter”
What is thyroid dysgenesis?
Thyroid gland does not develop or function properly; is absent, reduced in size or abnormally located.
Common in dogs, also seen in cats.
What is defective thyroid hormone synthesis?
Congenital enzyme deficiency that prevents synthesis of thyroid hormones
Quite rare
What is central (secondary) hypothyroidism?
Due to TSH deficiency
Can be pituitary or hypothalamic
Usually accompanied by decreased secretion of other pituitary hormones
Two types of hypothyroidism in adult animals:
Primary hypothyroidism -95% of cases
Central hypothyroidism -5% of cases
What is the pathogenesis of primary hypothyroidism?
Progressive autoimmune process which leads to lymphocytic infiltration and disappearance of thyroid follicles. (thyroid follicles are replaced by fibrous and adipose tissue)
The immune-mediated destruction is slow, clinical manifestations become evident after 75% of thyroid has been destroyed.
Sometimes the immunologic damage might involve other endocrine glands –> polyglandular failure syndrome
Pathogenesis of primary hypothyroidism in cats?
Iatrogenic hypothyroidism from radioiodine therapy or surgical thyroidectomy
Clinical manifestations of primary hypothyroidism in adult dogs:
-Weight gain
-Hypothermia, cold intolerance
-Appetite unchanged or reduced
-Coarse coat, alopecia
-Thickening of skin (myxedema)
-Bradycardia, weak peripherial pulse
-Persistent anestrus, loss of libido, testicular atrophy
-Letharhy
Hematological and biochemical findings in hypothyreosis
-Nonregenerative anemia
-Hypercholesterolemia
-Hypertriglyceridemia
-Mild hyperglycemia
Treatment of primary hypothyroidism
Lifelong thyroid hormone replacement therapy
T4 therapy will correct T3 also
Prognosis of primary hypothyroidism
Usually excellent with appropriate treatment and follow-up examinations
Cause and what happens in central hypothyroidims
-Thyroids are normal, but are deprived of stimulation by TSH
-Rare compared to primary
-Causes: tumor of pituitary or adjacent regions, head trauma, surgical removal of pituitary tumor
