Endocrine System (Thyroid) Flashcards
(38 cards)
Describe the differences between endocrine system and nervous system. (response, signaling, what they regulate)
Endocrine system: long-term slow response (slow acting hormone messengers travel through bloodstream to act on specific cell receptors)
Regulates: growth, development, reproduction (maintain homeostasis)
Nervous system: short-term quick response (fast acting electrochemical impulses (neurotransmitters) delivered by neurons (nerve cells) to synapses to target cells)
Regulates: muscles, glands
Explain two differences between endocrine and exocrine glands.
- Endocrine gland produce hormones, Exocrine gland produce nonhormonal substances
- Endocrine glands are ductless, Exocrine glands carry substances to membrane surface via ducts (e.g., sweat and saliva glands)
Endocrine glands are small in size, how many adults required to collect 1kg of endocrine tissues.
8-9 adults
What hormone does the hypothalamus release? (thyroid + reprod lecture series)
Thyrotropin-releasing hormones (TRH)
aka TSH releasing hormone
Reprod: Gonadotropin releasing hormones (GnRH)
What hormone does the anterior lobe of the pituitary gland release? (thyroid + reprod lecture series)
Thyroid stimulating hormone (TSH)
Reprod: gonadotropins
Follicle stimulating hormones (FSH)
Luteinizing hormones (LH)
What hormones does the thyroid gland release?
Tri-iodothyronine (T3)
Tetra-iodothyronine (Thyroxine) (T4)
Calcitonin (decrease calcium levels)
Name 2 synthetic estrogens
- BPA
- Paraben
can cause hastening of puberty, incr incidence of breast tumors, falling sperm counts, cryptorchidism)
Protein and peptide hormones are ___
while steroid and amine hormones are ___
List examples
Protein and peptide hormones are hydrophilic
while steroid and amine hormones are lipophilic
Protein: FSH, LH, TSH, insulin, glucagon, inhibin
Steroid: testosterone, progesterone, estrogen, Vit D (calcitriol)
Amines (tryptophan or tyrosine AA): T3, T4, melatonin
Exception hydrophilic amines: catecholamines - epinephrine, norepinephrine from adrenal medulla
What does the chemical nature of hormones determine?
Solubility, transport mechanism, and location of receptor
Hydrophilic: transported free in blood and bind to cell membrane receptor
Hydrophobic: bound to plasma protein, bind to intracellular receptors
*Does not determine specificity of communication - this is determined by presence of specific receptors on target cells
What are the two mechanisms of hormone imbalance?
- Excess/Deficiency in secretion of hormones
- Excess/Deficiency of target-cell responsiveness
What are the 2 mechanisms of hormone action after binding to receptor?
- Signal amplification: only low conc. of hormone required as signal gets amplified (therefore slow acting but long-lasting effect)
- Alter: functional and structural proteins, enzyme activation, transcription (trigger intracellular events)
E.g., alter channel permeability, act through second messenger system, activate specific genes
What is the mechanism in which hormones may inhibit its own release?
Negative feedback
Why are hormone levels reported in a range?
Secretion rates of hormones rhythmically fluctuate up and down as a function of time
Negative feedback control helps to regulate the fluctuations
Where is the thyroid gland located?
In neck, anterior to trachea, below the larynx
Two lobes connected by isthmus
Describe the histology of thyroid gland
Follicle Cells - synthesize globular protein, thyroglobulin (Tg) molecule, in endoplasmic reticulum/golgi complex. Tg is secreted it into the colloid
Colloid - extracellular space, where Tg attached with iodine is stored
Thyroid follicles - functional unit of thyroid hormone production (colloid + surrounding follicle cells)
Parafollicular (C) cells - calcitonin (dcr calcium levels)
What are the 2 basic ingredients in thyroid hormone (T3, T4) synthesis and how are they obtained?
- Tyrosine (AA synthesized by the body)
- Iodine (obtained from dietary intake) - must be reduced to iodide (I-) prior to absorption by SI
What are the 5 main steps involved in thyroid hormone synthesis?
- Iodide trapping
- Iodination
- Coupling
- Colloid resorption
- Thyroglobulin proteolysis
Where is thyroperoxidase (TPO) found?
What is the function of thyroperoxidase (TPO)?
TPO is a membrane-bound enzyme in follicular cells.
TPO oxidizes iodide (I-) to active iodide (I) in the follicular cell, before active iodide exits through a channel into the colloid
TPO is also responsible for attaching iodide to a tyrosine within the Tg molecule, in the colloid.
Tyrosine gets incorporated in the Tg molecule in follicular cells. Subsequently, tyrosine-containing Tg is exported in vesicles from follicular cells into the colloid by exocytosis.
How does iodide get transferred through the follicular cell into the colloid to attach to the Tyrosine-containing Tg molecule?
Iodide trapping: thyroid captures iodide (I-) from blood and transfers it into follicular cells by an iodide pump (Na+K+ pump)
==> Iodide transport Na+ into follicular cell down its conc. gradient, I- into cell against its conc. gradient
Iodination: Iodide (I-) is oxidised to active iodide (I) by membrane-bound enzyme, thyroperoxidase (TPO). Active iodide exits through a channel to enter the colloid. Within the colloid, TPO attaches iodide to a tyrosine within the Tg molecule ==> MIT, DIT
*1 iodine to tyrosine within Tg molecule: mono-iodotyrosine (MIT)
*2 iodines to tyrosine within Tg molecule: di-iodotyrosine (DIT)
Explain the coupling step in thyroid hormone synthesis.
Are these products attached to the Tg?
MIT + DIT = Tri-iodothyronine (T3)
DIT + DIT = Tetra-iodothyronine aka Thyroxine (T4)
Yes, these products remain attached to Tg by peptide bonds.
How does colloid resorption into follicular cells take place?
Follicular cells internalize a portion of the Tg-hormone complex by phagocytosing a piece of colloid (endocytosis of colloid)
How are thyroid hormones stored in the colloid (follicular lumen)?
Thyroid hormone bound to Tg for storage in the colloid
How is Tg removed from the iodinated products?
Thyroglobulin proteolysis occurs via lysosomes (lysosomal enzymes) attacking the engulfed colloid vesicle, and cleaving the peptide bond to remove Tg from the iodinated products
Where is iodinase found?
What is the function of iodinase?
Iodinase (like TPO) is found in the follicular cells
It removes the iodide from MIT and DIT, allowing the freed iodide to be recycled for synthesis of more hormones
