Thyroid Gland Flashcards
What are the 2 physiologically active thyroid hormones?
T3
T4
What are the 2 types of cells in the thyroid?
C (clear cells)
Follicular cells
What do clear cells secrete?
Calcitonin
What does calcitonin regulate?
Calcium
What tends to dominate the action of calcitonin?
PTH
What are thyroid follicles?
Structures surrounded by follicular cells
What is the centre of thyroid follicles filled with?
Colloid
What type of hormone are thyroid hormones?
Amine derived from tyrosine `
What do follicular cells manufacture?
The enzymes that make thyroid hormones as well as thyroglobulin
Where are thyroid hormone and thyroglobulin exported to?
The colloid
What do follicular cells concentrate?
Iodide
What does iodide combine with?
Tyrosine residues to form TH
Which enzyme catalyses the formation of TH>
Thyroid peroxidase
How does iodide enter follicular cells from the plasma?
via a Na+/I- transporter (symport they both travel in the same direction). The coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient.
How is iodide transported to the colloid?
Via the pendrin transporter
How is iodide transport inhibited?
by thiocyanates
What does the addition of one iodine to tyrosine form?
MIT (monoiodotyrosine)
What does the addition of two iodine to tyrosine form?
DIT
Diiodothyrosine
What does DIT + DIT form?
tetraiodothyronine or Thyroxine T4
What does DIT + MIT form?
triiodothyronine or T3
What happens in response to TSH?
Portions of the colloid are taken back up into the follicular cell by endocytosis. Within the cells they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release thyroid hormones.
Are T3 and T4 lipid soluble?
Yes
What do T3 and T4 bind to in the plasma?
Thyroxine-binding globulin
Plasma proteins
Where do T3 and T4 circulate?
In the plasma
What is the movement of TH from the colloid to the plasma under the control of?
TSH release from the PG
Where is TSH released from?
PG
How much TH circulates bound to plasma proteins?
99.8%
Which hormone is released from the hypothalamus to stimulate TSH production?
TRH
Why does T4 have a longer half life?
Because it has a higher affinity for thyroxine binding globulin
Does bound hormone exert an inhibitory effect on TSH and TRH?
no only free hormone does
What do TH receptors have a higher affinity for?
T3
Which TH is more physiologically active?
T3
Where do thyroid hormones bind?
To nuclear receptors in target cells
What is the effect of TH binding to receptors?
Change transcription and translation to alter protein synthesis
What are the effects of TH in the body?
Raises metabolic rate and promotes thermogenesis
increase hepatic gluconeogenesis
net increase in proteolysis
net increase in lipolysis
critical for growth (lack of TH results in retarded growth) anabolic
required for foetal brain development
What is congenital hypothyroidism?
Thyrodism from birth
How does TH affect growth?
Stimulates GH receptor expression
What happens to growth when TH is not present?
Receptors are not in adequate numbers and normal growth cannot occur
What can cause hyperthyroidism?
Graves disease
Thyroid adenoma
What causes graves disease?
(common) antibodies produced that bind mimic TSH and continually activate the thyroid gland. Increased release of TH switches off TSH release from anterior pituitary so [TSH] plasma is very low
What are the symptoms of hyperthyroidism?
Increased metabolic rate and head production
- weight loss
Increased protein catabolism - muscle weakness
Altered nervous system function
- hyper excitable reflexes
Elevated CVS function
- increased HR/contractile force
What is a main causes of hypothyroidism?
Hashmoto’s disease
Iodide deficiency
Idiopathic
What causes Hashmoto’s disease?
Autoimmune attack of thyroid gland
What are the symptoms of hypothyroidism?
- Decreased metabolic rate and heat production - weight gain/cold intolerance 2. Disrupted protein synthesis - brittle nails/thin skin
- Altered nervous system function - slow speech/reflexes, fatigue
- Reduced cardiovascular function - slow heart rate/weaker pulse
What is Goitre?
Enlargement of the thyroid gland
What is primary Thyroid disease?
When the TG is not working
What is secondary thyroid disease?
When the problem is with the PG
What is tertiary thyroid disease?
When the problem is in the hypothalamus
