The Thyroid Gland Flashcards
What s the shape of the thyroid gland?
Butterfly
Where does the thyroid gland lie?
Across the trachea at the base of the larynx.
What is the function of the thyroid gland?
Synthesises the thyroid hormones
What are the two physiologically active forms of the thyroid hormone?
T3 (triiodothyronine)
T4 (thyroxine)
The thyroid gland is mostly composed of two types of cell. Name them.
C cells
Follicular cells
What do C cells secrete?
Calcitonin
->calcitonin is a calcium regulating hormone
What is the function of the follicular cells?
Support thyroid hormone synthesis and surround hollow follicles
Describe the structure of thyroid follicles.
Spherical structures with walls made of follicular cells. The centre is filled with colloid
What is colloid?
Sticky glycoprotein matrix.
How many months worth of TH do the thyroid follicles contain?
2-3 months worth
Where in the thyroid follicles would you find the precursor form of TH?
Within the colloid
What do follicular cells manufacture?
The enzymes which make TH
Thyroglobulin- large protein rich in tyrosine residues.
Where do the enzymes and thyroglobulin created in the follicular cells get secreted into?
The colloid
What do the follicular cells concentrate?
Iodide
What does the concentrated iodide from the plasma combine with in the colloid to make the TH?
Tyrosine residues
How does the body get tyrosine and iodide?
Through the diet
How does iodide enter the follicular cells from the pladms?
Via a sodium/iodide transporter
->the coupling of sodium enables the follicular cells to take up the iodide against a concentration gradient
Now that iodide is in the follicular cell, how does it get into the colloid?
Via pendrin transporter
Basically iodide goes from plasma to follicular cells via Na/I transporter and then into the colloid via pendrin transporter
Enzymes which are exocytosed into the colloid alongside the thyroglobulin carry out what process?
Oxidation of iodide into iodine
After the iodide has been oxidised into iodine, what happens to the iodine?
Iodine is added to the thryoglobulin
What is it called if one iodine gets added onto tyrosine?
MIT (monoiodotyrosine)
If a second iodine is added onto tyrosine, what is this called?
DIT (diiodotyrosine)
MIT and DIT undergo conjugation where what is formed?
T3 aka triiodothyronine
If two DIT conjugate together, what is formed?
T4 aka tetraiodithyronine
In T3 aka triiodothyronine, how many:
a. tyrosines
b. iodines
a. 2 tyrosine
b. 3 iodine
In T4 aka tetraiodothyronine, how many:
a. tyrosines
b. iodines
a. 2 tyrosine
b. 4 iodine
Which hormones are the active form of thyroid hormone?
T3 and T4
RECAP- which type of hormone are thyroid hormones?
Amine hormones, however, they behave like steroid hormones
Why can we not store TH in their active forms of T3 or T4?
They are lipophilic and would cross the cell membrane and get into the blood.
TSH= thyroid stimulating hormone.
What happens in response to TSH which releases TH?
Portions of the colloid are taken back into the follicular cell by endocytosis.
Within the cells, they are packaged into vesicles containing proteolytic enzymes.
These enzymes cut the thyroglobulin to release TH
Are T3 and T4 lipid or water soluble?
Lipid soluble
As T3 and T4 are lipid soluble, they pass across the membrane and into the plasma. What happens to T3 and T4 when they are in the plasma?
They bind to plasma proteins, mainly thyroxine-binding globulin
Does thyroxine-binding globulin have a higher affinity for T3 or T4?
T4
->hint in the name, THYROXINE-binding globulin. Therefore there is more T4 circulating in the plasma than T3
How much of the TH in our plasma is physiologically active?
0.2%
-> the rest is bound to plasma proteins. However, only tiny concentrations of hormones are required by the body hence why this 0.2% is enough
Which has a longer half life, T3 or T4?
T4
->this is because of the greater affinity of the thyroxine-binding globulin for T4. Therefore, it releases it slowly meaning it has a longer half life
TH circulating in the plasma feeds back via negative feedback loops to switch off the release. Explain the pathway.
The circulating TH feeds back and antagonises the release of TSH from the anterior pituitary AND antagonises the release of TRH from the hypothalamus.
How much more T4 is protein bound than T3?
50x more
Which type of TH binds to 90% of the TH receptors?
T3
->there is a higher affinity for T3 by the TH receptor than T4
So therefore, which TH is most physiologically active?
T3
How does T4 become deiodinated to form T3?
By deiodinase enzymes
T4 can be deiodinated in two places, where?
Half is deiodinated in the plasma, half in the target cells
What are those with hypothyroid supplemented with?
T4 as T3 has too short a half life.
However, T3 levels remain normal as the deiodinase enzymes convert T4 into T3
List some of the main stimulating factors which release TRH from the hypothalamus.
Cold, exercise and pregnancy
Glucocorticoids and somatostatin are both inhibitory when it comes to the release of TH.
How is somatostatin inhibitory?
Inhibits TSH
Glucocorticoids and somatostatin are both inhibitory when it comes to the release of TH.
How are glucocorticoids inhibitory?
Inhibit TSH and the conversion of T4 into T3
How do TH alter protein synthesis?
They bind to receptors in the target cells and change transcription which alters protein synthesis
TH raise metabolic rate. What happens as a result?
Promotes thermogenesis (increase in temperature of the body)
How does TH promote thermogenesis?
By promoting futile cycles of simultaneous catabolism and anabolism, which produces heat energy
Hyperthyroidism?
Over activity of the thyroid gland
What are the main two causes of hyperthyroidism?
Graves Diseases- common
Thyroid adenoma- less common
What happens in Graves disease?
Body produces antibodies which mimic the action of TSH so there is continual activation of the thyroid gland
How does the thyroid gland change in those with graves disease?
Increases in size by 2-3x due to hyperplasia
How can a thyroid adenoma cause hyperthyroidism?
The tumour secretes hormones
What are the symptoms of hyperthyroidism?
Weight loss/heat intolerance
Muscle weakness
Hyperexcitable reflexes and psychological disturbances
Increased HR, cardiac failure
Why is there weight loss/heat intolerance in those with hyperthyroidism?
Increased metabolic rate and heat production
Why is there muscle weakness in those with hyperthyroidism?
Increased protein catabolism
Why is there hyperexcitable reflexes and psychological disturbances in those with hyperthyroidism?
Altered functioning of the NS
Why is there increased HR and potentially cardiac failure in those with hyperthyroidism?
Elevated cardiovascular function as TH is permissible to epinephrine on the beta receptors
What happens in hypothyroidism?
Thyroid gland is underperforming
List some of the causes of hypothyroidism.
Hashimoto’s disease
Deficiency in dietary iodine
What happens in Hashimoto’s disease?
Autoimmune attack of the thyroid gland preventing the release of thyroxine and triiodothyonine
List some food sources of iodine :)))
Milk
Fish
Seafood
Seaweed
What are some of the symptoms of hypothyroidism?
Weight gain, cold intolerance
Brittle nails, thin skin
Slower reflexes, fatigue
Reduced CVS function- slow HR and weak pulse
What causes weight gain and cold intolerance in those with hypothyroidism?
Decreased metabolic rate and heat production
What causes the brittle nails and thin skin in those with hypothyroidism?
Disrupted protein synthesis
What causes the slow reflexes and fatigue in those with hypothyroidism?
Altered functioning of NS
Goitre?
Significant enlargement of the thyroid gland
What can cause goitre formation relating to hypothyroidism?
Increased trophic action of TSH on thyroid follicular cells
What can cause goitre formation relating to hyperthyroidism?
Graves disease- autoimmune
If someone is deficient in iodine, what can they not produced?
T3/4
If there’s not enough T3/, which hormone is released from the hypothalamus?
TRH
->this can cause the thyroid gland to enlarge
