Case 9 Flashcards
1
Q
What are the hormones the thyroid gland secretes?
A
- Thyroxine
- triiodothyronine
- calcitonin
2
Q
Which thyroid hormoen is more potent?
A
triiodothyronine
3
Q
What is the function of calcitonin
A
calcium metabolism
4
Q
How are T4 and T3 transported in the blood?
A
- T4 and T3 are bound to plasma proteins
- on entering the blood the T4 and T3 combine immediately with several of the plasma proteins which are synthesized by the liver
- T4 and T3 are released slowly to tissues
5
Q
How do thyroid hormones affect carbohydrate metabolism?
A
- stimulation of carbohydrate metabolism
- rapid uptake of glucose by cells
- enhanced glycolysis
- enhanced gluconeogenesisi
- increased rate of absorption from GI tract
- increased insulin secretion
6
Q
How do thyroid hormones affect fat metabolism?
A
- Lipid mobilised rapidly from fat tissue decreasing fat stores of the body to a greater extent than almost any other tissue element
- increases the free fatty acid concentration in the plasma and accelerates oxidation of FFA by cells
7
Q
How do thyroid hormones affect plasma and liver fats?
A
- Decreases level of cholesterol, phospholipids and triglycerides
- increases rate of cholesterol secretion in the bile and consequent in faeces
- Thyroid hormone induces increased numbers of LDL receptors on the liver cells leading to rapid removal of LDL from the plasma by the liver and subsequent secretion of cholesterol in the lipoproteins by liver cells
8
Q
How do thyroid hormones affect the basal metabolic rate?
A
- Thyroid hormone increases metabolism in almost all cells of the body, excessive quantities of the hormone can occasionally increase the basal metabolic rate
- when there are no thyroid hormones basal metabolic rate drops
9
Q
How do thyroid hormones affect the muscles?
A
- Effect on the function of the muscles - slight increase in thyroid hormone usually makes the muscles react with vigor but when the quantity of hormones becomes excessive muscles become weakened because of excess protein catabolism, lack of thyroid hormone causes the muscles to become sluggish and relax slowly after contraction
- Muscle tremor - caused by increased reactivity of neuronal synapses in the areas of the spinal cord that control muscle tone, the tremor is improtant for assessing the degree of thyroid hormone effect on CNS
10
Q
What effects does TSH have on the thyroid gland?
A
- Increased proteolysis of the thyroglobulin stored in follicles
- increased activity of the iodide pump
- increased iodination of tyrosine to form thyroid hormones
- increased size and increased secretory activity of the thyroid cells
- increased number of thyroid cells
11
Q
Describe the mechanism of TSH binding to thyroid gland?
A
- TSH binds to receptors on bassal membrane of the thyroid cell
- activates adenyl cyclase in the membrane increasing formation of cAMP inside the cell
- cAMP acts as a second messenger to activate protein kinase causing phosphorylation in the cell
- this results in increased secretion of the thyroid hormones and prolonged growth of glandular tissue
12
Q
Describe the hypothalamic pituitary axis?
A
- The hypothalamus releases TRH (thyrotropin hormone, which travels and acts on the anterior pituitary gland
- The anterior pituitary releases TSH (thyroid stimulating hormone)
- TSH stimulates the thyroid gland to make 2 hormones T3 and T4 which act on periphery tissues and also on the anterior pituitary gland as a negative feedback loop
13
Q
Describe the synthesis of thyroid hormones?
A
- TSH comes through the circulation stimulating the thyroid gland
- stimulates production of the thyroglobulin
- iodine is concentrated in the thyroid, iodine is added to the thyroglobulin by a process termed organification to make a xomplex of T4
- The complex fo T4 returns to the colloid into the cell to be recycled and then secrete the hormone
- thyroid gland stores the hormone
- if the hormone is infected and inflames it dumps out all of the stored thyroid hormone and this leads to thyrtoxicity
14
Q
What are colloids?
A
Stores of thyroid hormone, they cause the thyroid gland to become nodullar
15
Q
Describe the synthesis of T4 and T3 from tyrosine
A
- T4 and T3 are made from tyrosine amino acid
- An iodine molecule is added to tyrosine to form monoidotyrosine
- another iodine is added to form diodotyrosine
- when a 3rd iodine is added this forms T3 addition of another forms T4
- removal of an iodine from T3 forms an inactive form dioodothyronine T2
- Removal of iodine from T4 forms triiodothyronine forms an inactive form T3
16
Q
How is thyroid hormone metabolised in peripheral tissues and in the circulation?
A
- Thyroid hormoens are released from the gland into the circulation and peripheral tissues
- T4 loses an iodine molecule via selenodeiodinase enzyme 1 and becomes the active form T3 and does its biological activity
- T4 may lose iodine via selenodeiodinase 3 and becomes the inactive reverse T3 which is excreted
- Selenodeiodinase 3 may act on T3 and form an inactive T2
