Thyroid Gland Flashcards
_____________ stimulates thyrotropes in the hypophysis/pituitary gland causing thyroid stimulating hormone (TSH) release
Thyrotropin Releasing Hormone (TRH)
Thyrotropin Releasing Hormone (TRH) stimulates thyrotropes in the hypophysis/pituitary gland causing
thyroid stimulating hormone (TSH) release
TSH stimulates _______ and _________ secretion from the thyroid gland
T3 & T4
T3 is also known as
Triiodotyronine
T4 is also known as
Thyroxin
Is T3 or T4 more biologically active?
T3 - but most tests are for T4 because potency period of T4 is longer
What inhibits TSH release from the pituitary?
T3 & T4
What inhibits TRH release from hypothalamus?
Excess T3 & T4
What happens when there is excessive stimulation of the thyroid gland by TSH?
Thyroid enlargement or goiters
What is the functional unit of the thyroid gland?
A thyroid follicle, which is made up of thyroid follicular cells & colloid (protenacious material present in the thyroid follicle)
Thyroid hormones are __________
Iodine containing amino acids
Iodination of tyrosine residues of thyroglobulin (TGB) leads to the formation of
Monoiodotyrosine (MIT) and diiodotyrosine (DIT)
Difference between monoidotyrosine (MIT) and diiodotyrosine (DIT)?
How many iodines are present. MIT has 1 Iodine at one site and DIT has both iodine binding sites filled (fully saturated)
T3 results from
Oxidative condensation of MIT and DIT
T4 results from
Oxidative condensation of two DIT molecules
Thyroid hormone is a hormone synthesized by
Iodination of the amino acid tyrosine
Under the influence of TSH, thyroid cells take iodide from the blood circulation and then
Oxidize it to iodine by thyroporeoxidase & enters thyroid colloid
Thyroglobulin (a protein) is synthesized in the
Thyroid follicular cell
Thyroglobulin enters the colloid along with iodine by the process of
Exocytosis
Thyroglobulin is a protein with _______ tyrosine amino acids
134
Each tyrosine aa has _____ iodination sites
2
Thyroglobulin containing T3 and T4 will be endocytosed into
Thyroid follicular cells, then thyroglobulin undergoes hydrolysis, releasing T3 and T4 into systemic circulation
Hypothalamus secretes TRH, which acts on the
Pituitary gland to secrete TSH
TSH acts on the thyroid gland to secrete
T3/T4
T3/T4 will reach the target cells via
Blood vessels
At the target cells, T3/T4 interacts with its nuclear receptor and this receptor-hormone complex will
Modify DNA or gene transcription by binding to the TRE (thyroid response element) of DNA
Resulting mRNA after TRE/hormone binding does what
Is translated to proteins required for various physiological functions
T3 & T4 exert negative feedback on
TSH and TRH release
TH-MSD stands for
Thyroid gland hyperplasia and concurrent musculoskeletal deformities
TH-MSD can occur in foals, characterized by
Thyroid gland enlargement along with deformities of the musculoskeletal system
The fetus needs what hormones
Thyroid hormones and fetal thyroid gland synthesizes these hormones under normal circumstances
Is the placenta a barrier to T3/T4?
YES! That’s why the fetal t3/t4 are important & gland must be functional for proper development
What can lead to TH-MSD?
Dietary issues of the mare (low iodine and high nitrate) of the mare. Iodine & nitrates can pass through the placenta.
Why does high nitrate and low iodine lead to fetal TH-MSD?
Iodine is an absolute requirement for TH synthesis, and nitrates compete for iodine utilization in the thyroid gland
Hyperthyroidism affects
Cardiac function. Thyroid hormones increase number and affinity of B-adrenergic receptors on the heart, leading to an increased heart rate and a positive inotropic effect
Thyroid hormones ____________ the myosin content
Increase
Increased myosin content leads to
Increased force of contraction and hypertrophic cardiomyopathy (HCM)
In hypertrophic myopathy (HCM) there is thickening of
Ventricular septum & ventricular wall, which reduces ventricular chamber space & reduced filling. All these changes can lead to heart murmur in affected cats
Thyroid hormone is needed for normal _______ gland function, _________, and normal keratinization
Sebaceous gland function, hair growth, and normal keratinization
improper or poor hair coats will be seen in animals suffering from
Thyroid hormone deficiency
Hyperthyroid cats are generally
Nervous and anxious
Extra nervousness and anxiety in hyperthyroid cats is due to
Imbalance in inhibitory and stimulatory neurotransmitters as well as the imbalance in calcium, magnesium, and potassium metabolism
__________ism is more common in cats while ___________ism is more common in dogs
Hyperthyroidism - cats, hypothyroidism - dogs
what is the most common endocrinopathy in cats?
Hyperthyroidism
What is the most common cause of hyperthyroidism in cats?
Functional adenoma (adenomatous hyperplasia), which accounts for 80% of cases
What are causes of hyperthyroidism that are NOT the most common (functional adenoma)?
Functional adenocarcinoma (5%)
Over supplementation of thyroxin
What is the common signalment for hyperthyroidism in cats?
Middle-aged to older cats
Clinical signs of hyperthyroidism?
Enlarged neck mass
dyspnea/cough
vomiting
weight loss
nervousness
PU/PD
diarrhea
heart murmur
T4 above normal physiological levels
What is primary hypothyroidism?
Problem at level of thyroid gland
What are the 2 types of primary hypothyroidism?
Congenital vs acquired
Congential primary hypoplasia
Cretinism: aplasia or hypoplasia of the gland
Acquired primary hypothyroidism
-Lymphocytic thyroiditis (50%) of canine cases) due to the development of autoantibodies against thyroglobulin
- idiopathic follicular atrophy
-neoplasia (thyroid gland destruction)
What is the most common kind of primary hypothyroidism?
Lymphocytic thyroiditis
What is secondary hypothyroidism?
Problem @ the level of the pituitary
Examples of secondary hypothyroidism
Hypopituitarism
Pituitary neoplasia
Tertiary hypothyroidism is what
Problem @ level of hypothalamus
Example of tertiary hypothyroidism
Failure of hypothalamus to secrete TRH
Hypothyroidism clinical signs
-Lethargy and cold intolerance
-weight gain
-dermatologic hyper pigmentation, alopecia, sebaceous gland atrophy
-nervous system: peripheral nerve dysfunction
Muscle atrophy
-infertility
-vomiting
- cardiac signs ( different cue card)
Hypothyroidism cardiovascular clinical signs
-Sinus bradycardia (T3 is required for expression of B-adrenergic reponse)
- normochromic normocytic anemia
- hypercholesterolemia
- hypertriglyceridemia
What is considered the most reliable and definitive test for diagnosing hypothyroidism ?
Thyroid stimulating hormone (TSH) reponse test
Describe how the TSH response test works
Stimulating thyroid gland with exogenous TSH increases T4 above baseline in normal dogs
- dogs with deficiencies in thyroid function will not have a function will not have an increase
- 0.1 units of TSH/kg is administered (maximum 5 IU per dog). Serum samples are taken before and 6h after administration
Interpretation of TSH response test
Normal response: T4 level increases >50 nmol/L
Primary hypothyroidism: minimal increase in T4 concentrations; remains below normal baseline
T4 medication must be must be terminated for 6-8 weeks before TSH test
