Thyroid Gland Flashcards
What enzyme converts T4 into T3?
__% converted in periphery
__% directly secreted as T3 from the Thyroid Gland
Deiodinase
90
10
Functional unit of parathyroid gland
Thyroid follicle
Parafolicular cells that produce Calcitonin
C cells
Colloid is composed of newly synthesized thyroid hormones-attached to ________.
Thyroglobulin
Clinical states associated with reduced conversion of T4 to T3.
Fasting
Medical and Surgical stress
Catabolic Disease
Process of binding Iodine to Thyroglobulin
Organification
Steps of Thyroid Hormone synthesis catalyzed by peroxidase
Transport of I- into follicular lumen and oxidation to I2 Organification of I2 (I2+TG=MIT; DIT) Coupling Reaction (DIT+DIT=T4; DIT+MIT=T3)
Inhibitor of peroxidase
PTU (Propylthiouracil)
Inhibitors of Na+/I- symporter (NIS)
Perchlorate
Thiocynate
Inhibition of which step of thyroid hormone synthesis results in Wolff-Chaikoff effect?
Organification of Iodine
What can result in inhibition of organification and therefore in Wolff-Chaikoff effect?
High levels of I-
Iodine is stored iodinated as tyrosine of ___.
TG
Enough hormone is stored in the follicular colloid as iodinated TG to last for ____.
2-3 months
What is used to assess the activity of thyroid gland?
Radioactive Thyroid uptake
Normal Iodine uptake
15% at 6 hr, 25% at 25hr
Hyperthyroid Iodine uptake
> 60% at 24 hr
Hypothyroid Iodine uptake
<5% at 24 hr
Thyroid hormones circulate in the bloodstream either bound to proteins (__%), or free (__%).
99; 1
Thyroid hormone binding proteins
Thyroxin-binding globulin (TBG)-70%
Transthyretin (TTR)- 10-15%
Albumin - 15-20%
Where is TBG synthesized?
Liver
T/F: TBG has higher affinity to T4 that to T3.
True
What test is used to indirectly assess the circulating levels of TBG?
T3 Resin Uptake Test
If T3 resin uptake is high that means levels of TBG is ____.
Low
If T3 resin uptake is low that means levels of TBG is ____.
High
In hyperthyroidism T3 resin uptake is Increased or decreased?
Increased
In hypothyroidism T3 resin uptake is increased or decreased?
Decreased
T/F: even though levels of TBG are elevated during pregnancy, levels of free, physiologically active thyroid hormones are normal.
True (“clinically euthyroid”)
Hepatic failure leads to hyperthyroidism at first because unbound T3 and T4 levels go up, but later it leads to hypothyroidism due to decreased synthesis of T3 and T4 via _______ __________.
Negative feedback
Percentage of T3 bound to TBG vs percentage of T4 bound to TBG
T3: 99.5%
T4: 99.98%
How are T3 and T4 transported from blood into cytosol in thyroid or periphery?
MCT8/10 transporters
How many types of deiodinases are there?
Two: type 1 & 2 (different tissue distributions and affinities)
T/F: T3 receptors are in cytosol
False: in nucleus
Effect of TH on Growth
Growth
Bone maturation
Effect of TH on Nervous System
CNS maturation
Effect of TH on BMR
Increase Na+/K+ ATPase
Increase O2 consuption
Increase Heat production
Increase BMR
Effect of TH on Metabolism
Increase glucose absorption Increase Gluconeogenesis Increase Glycogenolysis Increase Lipolysis Increase Protein Synthesis and degradation (net catabolic)
Effect of TH on Cardiovascular System
Increase cardiac output
Up regulation of beta-adrenergic receptors
Effect of TH on GI
Increase motility
Proteins whose synthesis is unregulated by TH
- Na+/K+ ATPase
- Transport proteins
- betta-1 adrenergic receptors (cardiac muscle)
- Myosin (cardiac muscle)
- Ca2+ ATPase
- Lysosomal enzymes
- Proteolytic enzymes
- Structural proteins
T/F: Cholesterol and TG levels in blood are inversely correlated with TH levels
True (higher TH, lower cholesterol)
TH is required for conversion of carotene to vitamin A, therefore hypothyroidism can lead to ________.
Blindness
____days before activity of T4 begins
____days before activity of T4 reaches maximum
Activity of T4 persists _____wk
2-3
10-12
6-8
Are actions of T3 slower or more rapid than that of T4?
more rapid
_____ hr before activity of T3 begins
_____ days before activity of T3 reaches maximum
6-12
2-3
Is Thyroid Hormone more sensitive to stimulation by parasympathetic or sympathetic nervous system?
Sympathetic
High levels of TH increases the number of _-__________ receptors in cardiac muscle cells.
beta1-adrenergic
Effects of excess TH on skin
Sweating
Effect of TH deficiency on skin
Dry
Mixedema
Effect of excess TH on CNS
Agitation
Anxiety
Difficulty concentrating
Hyperreflexia
Effect of TH deficiency on CNS
Cretinism (congenital) In adults: listlessness slowed movement somnolence impaired memory decreased mental capacity
Which disease is an example of primary hyperthyroidism?
Graves disease
Example of secondary hyperthyroidism
TSH-secreting pituitary tumor
Presence of what in the serum helps distinguish Graves Disease from adenoma of pituitary thyrotrophs (TSH-secreting tumors)?
TSI (thyroid-stimulating immunoglobulins)
How do TSIs cause hyperthyroidism in Graves disease (increase of T3 and T4 in the blood)?
They bind to TSH receptors in the absence of TSH and so TSH receptor is ON all the time.
TSH levels in Graves disease high or low?
Low (negative feedback)
Which disease is an example of hypothyroidism?
Hashimoto’s thyroiditis
Effect of excess TH on bone
Osteoporosis
Effect of TH deficiency on bone
Stunted growth
In Hashimoto’s Disease TH synthesis is impaired by antibodies against _________, leading to decreased T3 and T4 secretion.
Thyroglobulin or TPO
In Hashimoto’s Thyroiditis TSH levels are _____ due to loss of negative feedback. This has trophic effect and leads to _____.
high; Goiter
Possible causes of Cretinism (example of hypothyroidism)
- Iodine deficiency
- Maternal intake of anti-thyroid medications
- Impaired development of thyroid gland
- Inherent deficit in synthesis of thyroid hormones
Hypothyroidism due to iodine deficiency results in what processes?
- Transient decrease in synthesis of T3 and T4
- TSH levels elevated
- Goiter