thyroid gland Flashcards
location of the thyroid gland
below the larynx
isthmus of the thyroid gland
two lobes are positioned on either side of the trachea and are connected anteriorly by a medial mass of thyroid tissue
largest of the pure of the endocrine gland
thyroid gland
hormones secreted by the thyroid gland
thyroxine (T4)
triiodothyronine (T3)
secrete thyroxine (T4) and triiodothyronine (T3)
thyrocytes, the cuboidal-shaped epithelial cells lining the thyroid follicles
follicular cells
secrete a hormone known as calcitonin or thyrocalcitonin
parafollicle cells or C cells
T4 thyroxine
as the principal (most abundant) but less active
T3 (triiodothyronine)
as the most active but short-life
how do T3 and T4 travel in blood?
bound to plasma proteins
thyroxine-binding globulin, or TBG
T4 and T3 are made by tyrosines after what process
iodinazation
Major protein component of follicle colloid secreted by follicular cells contains
Thyroglobulin (TG)
Tyrosines manufactures TH derived from
Thyroglobulin (TG)
steps of synthesis of thyroid hormone
1.Thyroglobulin (TG) are secreted into the colloid of the follicle
2.
*The thyroid follicles actively accumulate iodide from the blood and secrete it into the colloid.
*Once the iodide has entered the colloid, it is oxidized to iodine.
3. iodines are attached to tyrosines
4. iodine + tyrosine = monoiodotyrosine MIT
attachment of iodines produces
5. within the colloid, enzymes modify the structure of MIT and DIT and couple them together
6. 2 DIT are coupled together = T4
1 MIT + 1 DIT = T3
7. stimulation of TSH = cells of the follicle take up small volume of colloid by pinocytosis
8. hydrolyze TG by protease to free the T3 nad T4
9. serete T3 and T4 in the blood
2 diiodotyrosine (DIT) =
T4 tetraiodothyronine or thyroxine
1 monoiodotyrosine MIT + 1 diiodotyrosine DIT =
T3 triiodothyronine
which one is lipophilic, T4 or T3?
T4
can directly diffuse through plasma membrane of target cells into the cytoplasm
T4
3 step mechanism of action of T4, T3 on target cells
- T4 is converted by deiodinase to T3
- T3 non-specific binding proteins to enter the nucleus and binds to nuclear receptor
- hormone receptor complex binds to a specific area of DNA activating specific genes transcription and protein synthesis
The full activation of targeted gene expression need the cooperation of the
thyroid hormone receptor (TR)
RXR, retinoid X receptor for the vitamin A derivative 9-cis-retinoic acid
when TR + RXR they form a
heterodimer can bind to the DNA to activate the hormone response element for thyroid hormone
effects of thyroid hormone
cell respiration increase and ATP production in mitochondria in most tissues of the body
increases basal metabolic rate
increases lipolysis and protein breakdown
increases heat production
increases cellular work
increases body temperature
effects of TH on skeletal muscle
Stimulate synthesis of proteins (e.g. myosin heavy chains = neuromuscular activity
favors fast-twitch fibers (white type 2) (Myh2)
Hypothyroidism
decreased ATP production = muscle weakness
Hyperthyroidism
increased contration = msucle fatigue = muscle degeneration (thyrotoxic myopathy)
effects of TH on heart
increased heart rate and contractility
expression of alfa and beat receptors
expression of alfa-mysosin heavy chain and Ca channels in myocardium
increased O2 consumption and increased cardiac output
effects of TH on nervous system
brain development
t/f TH receptors are found in brain cells
true
Lack of TH during development leads to
cretinism
Effects of TH on Catecholamines
Increase the sensitivity of norepinephrine and epinephrine by:
Expression of alfa1- and beta receptors
increased metabolism, excitability and sweating
TH secretion is augmented by
- TSH (=thyrotropin) from anterior pituitary (directly)
- TRH (=thyrotropin-releasing hormone) from hypothalamus (indirectly, through TSH)
Actions of TSH (thyroid stimulating hormone=thyrotropin)
Activation of all steps in T3 and T4 synthesis/secretion pathway.
Promote proliferation of follicles
what causes Goiter?
enlarged thyroid gland
due to iodine deficiency
in the absence of dietary iodine
thyroid cannot produce adequate amounts of T3 and T4
lack of negative feedback inhibition causes abnormally high levels of TSH secretion in pituitary,
stimulates the abnormal growth of the thyroid follicle to compensate the inadequate TH production, leading to a normal TH level in most occasions.
Toxic goiter or Grave’s disease is caused by
TH (T3 and T4) is very high and TSH is reduced
Caused by Autoantibody (TSI: thyroid-stimulating immunoglobulin) binding to TSH receptor
LEADS TO HYPERTHYROIDISM
Accompanied by exophthalmos, or bulging eyes, due to edema in the orbits, accompanied by weight loss, nervousness, irritability, and an intolerance to heat. There is also a significant increase in cardiac output and blood pressure.
TOXIC GOITER
COMMON CAUSES of hyperthyroidism in cats
A hormone-producing, benign thyroid tumor
common clinical signs of hyperthyroidism in cats
Weight loss: despite an increased appetite (most common sign).
Hyperactivity: increased activity, restlessness; aggressive or “cranky” behavior,
Palpable goiter
Unkempt coat: poor hair coat
causes of hypothyroidsim in dogs
caused by one of two diseases: lymphocytic thyroiditisor idiopathic thyroid gland atrophy.
Common clinical signs of hypothyroidism in dogs
Obesity: weight gain without an increase in appetite
Skin disease: loss or thinning of fur, dull hair coat, excess shedding or scaling
Lethargy: reduced activity and exercise intolerant
Cold intolerance
