Jackson-Endocrine System- Thyroid Flashcards
Which cells of the thyroid regulate the production of the two thyroid hormones?
The epithelial or follicle cells
T/F thyroid hormones are lipophilic? What does this imply?
True
It implies that they can’t be stored in secretory vesicles
What are the steps in thyroid hormone synthesis?
- Iodide is transported across the basal side (blood stream side) of the follicle cells via an Na+/I- transporter
- Iodide diffuses down its concentration gradient across the apical membrane into the colloid of the follicle
- The follicle cells also synthesize a large, tyrosine-rich protein called thyroglobulin (TG) as well as the enzyme thyroid peroxidase. Both proteins are exocytosed across the apical membrane into the colloid
- In the colloid (which is ECF), iodide is oxidized by thyroid peroxidase and then linked to the thyroglobulin. Binding iodine to thyroglobulin maintains the concentration gradient so that I- can continue to come in.
If one Iodide is added we get monoiodotyrosine (MIT)
If two iodides are added we get diiodotyrosine (DIT) - DIT’s combine with either other DIT’s to give us T4 or MIT’s to give us T3
Where can thyroid hormone be stored?
The colloid
Secretion of the thyroid hormones is controlled by what? Where does it come from? What controls that? Where does that come from?
Thyroid stimulating hormone from
Anterior pituitary controlled by TRH (thyroid releasing hormone) which is released from the hypothalamus
How is thyroid hormone secreted? What are the steps?
- In response to TSH from the anterior pituitary, droplets of colloid containing thyroglobulin (TG) and T3/T4 are pinocytosed into the follicle cells (in other words thyroid hormone and TG are sucked into follicle/epithelial cells)
- The droplet fuses with a lysosome containing enzymes that cleave the TG from T3 or T4
- T3 and T4 are released in the cytoplasm by lysosomal hydrolysis
- T3 and T4 diffuse into capillaries. Amino acids from the degraded TG are recycled into new TG
Thyroid hormone has negative feedback actions that regulate its own secretion at the level of the hypothalamus and pituitary
T/F only 10% of the thyroid hormone that is secreted from the thyroid gland is T4?
False, this is true, but for T3
Which thyroid hormone is 10 times more biologically active than the other?
T3
80% of the T4 that is secreted is converted into T3 in the liver and kidney. Why is this done?
This helps maintain the concentration gradient for free iodide between the blood and colloid to make sure it’s available for TH synthesis
T/F- receptors for TH are in the nuclei of most cells of the body?
True
What happens to receptors that are occupied by TH?
They dimerize and form a DNA binding protein that regulates gene transcription
T/F unbound TH receptors can bind DNA and inhibit transcription?
True
The principle effect of thyroid hormone is what? What are some examples of this?
To stimulate cellular metabolism Some examples are: increase NA/K ATPase activity increase synthesis of respiratory enzymes increase substrate availability increase cellular heat production effects on mitochondria
What are some additional effects of TH?
Upregulation of Beta-adrenergic receptors- increases sensitivity to catecholamines in endocrine and nervous system
Regulates the production of growth hormone
it’s also important in CNS development and function
T/F worldwide, thyroid disease is the most common endocrine disease?
True
What his hypothyroidism?
it’s most often caused by a primary defect in the thyroid gland and is a thyroid hormone deficiency due to lack of sufficient intake of iodine
Why does a goiter develop when there is a lack of iodine in the diet?
Without iodine there is insufficient TH production. Without enough TH, there won’t be a negative feedback mechanism that decreases that release of TRH and TSH (from hypothalamus and anterior pituitary). Consequently TRH and TSH are secreted almost continually and the goiter grows and grows
How can an iodine deficiency be reversed?
By adding iodine to the diet
What are the consequences of an iodine deficiency during prenatal development?
moderate deficiency to cretinism, miscarriage, stillbirth. Growth and neural development are impaired
What are some other primary defects leading to hypothyroidsim?
autoimmune thyroiditis
damage or destruction of the gland
dysfunction associated with other illness
With these, the thyroid gland can’t make TH even if there is sufficient iodine
Primary effects are in regard to the thyroid gland itself. What do secondary effects refer to?
things that act on the thyroid IE TRH or TSH from the hypothalamus or anterior pituitary, so thyroid function is fine, its just not getting the right messages
What are some symptoms of hypothyroidism?
These symptoms are more severe as the case of hypothyroidism is more severe:
abnormal circulating concentrations of TH and TSH
Goiter development
Cretinism and mental retardation if during prenatal development
sensitivity to cold and slight weight gain
fatigue, reduced blood flow, changes in skin tone, sluggish gastrointestinal motility and mental function
sever bloating due to accumulation of stuff in the SCF
What is hyperthyroidism?
Results from having too much TH (far less common than hypothyroidism)
What can cause hyperthyroidism?
primary defects: thyroid tumors that produce TH without regulation or inflammation of the gland that leads to excess production
