Thyroid Gland Flashcards
What connects the two lobes of the thyroid?
Central Isthmus
Where is the thyroid located?
In the neck, in front of the upper trachea and lower larynx. Below Cricoid cartridge
How wide is the thyroid and what shape is it?
2-3cm across and butterfly shape
What 2 nerves lie close to the Thyroid?
RECURRENTLARYNGEAL
& external branch of SUPERIOR LARYGEAL so thyroid surgery puts larynx and speech at risk
How vascularised?
Highly.
3 arteries and 3 veins
- superior, middle and inferior
What are the 2 types of cells present?
Follicular and Parafollicular (C-cells)
Where is the Parafollicular cells found?
The connective tissue
How are the follicular cells arranged?
Follicles separated by connective tissue.
More precisely, spherical shapes with follicular epithelia. And lumen containing a protein called COLLOID with THYROGLOBULIN which is large and rich in Tyrosine
What hormones found in follicular cells?
T4 Tyroxine and T3 Tri-idothyronin
What are these hormones synthesised from?
From tyrosine residues on thryoblulin.
It is iodinated.
What is hormone secretion regulated by?
FSH levels
Which hormone is more potent?
T3 is more potent than but shorter half life
But more T4 is produced.
What happens to T4 in peripheral tissue?
It converts to T3 in deiodination tissue hence is more active.
Removal of 5’ iodine produces inactive reverse T3 which binds to receptors without stimulating them
What hormone is found in the Parafollicular cells?
Calcitonin - a polypeptide hormone involved in calcium metabolism
Describe effects of TH on Nervous System
Important for development in children and function in adults. Increases: Myelination of nerves and development Speed of reflexes Mental activity (memory and alertness)
Describe effects of TH on Cardiovascular
Increases cardia output
DIRECT effect on heart
Describe effects of TH on Skin & Subcataneous Tissue
Increases turnover of proteins and glycoproteins
Describe general effects of TH in the body
Increases BMR, #mitochondria, O2 consumption, heat production, nutrient utilisation
Stimulate metabolism, increasing lipolysis, glygenolysis, glycolysis, proteolysis
Promote normal growth and development of tissues
Increasing synthseis of certain proteins
Increase responsiveness of Sympathetic NS & various hormones such as noradrenaline
How is TSH released?
HYPOTHALMIC- PITURATORY-THYROID AXIS
- Thyrotropinin releasing hormone released from Hyp.
- Travels down HYPOTHALAMIC- hypophysis all portal vein to Anterior Pit. Gland binding to G9/11 receptors called PLC.
- The complex signals the release of Ca2+ which triggers exocytosis of TSH into the systemic circulation.
What does TSH do at the Thyroid?
- TSH binds to the basolateral membrane of the follicular cells
- Binding triggers proteolytic cleavage of Thyroglobin to release T3 & T4 which diffuses into circulation.
- TSH also has tropic effects, increasing vascular its and the size and #of follicles hence can produce a goitre
How is T3 & T4 produced?
- Iodine trap in the colloid. High [iodine], from the diet.
Na/K pump, transports 3Na+ outside the Follicular cell into the Intersticium producing a Na gradient. Na then couples with Iodine through a channel protein. The iodine then travels through another channel km the follicular lumen. - Iodine activated by peroxidase associates with thyroglobulin
- The Iodothyronine proteins stored as colloid
T3 binding to receptors?
T3 binds to hormone-binding domains of receptors causing a conformational change unmasking DNA binding domain. The hormone-receptor complex increases transcription.
How is T3 & T4 transported?
The hormones are hydrophobic hence most must be bound to THYRONINE BINDING GLOBULIN in the blood. A little is bound to ALBUMIN & PREALBUMIN (TBPA)
Less than 1% is free
How does osteogenesis affect TH?
It increases the synthesis of TBG and decreases the clearance of Iodothyronines as they are bound.
Hence high levels of oestrogen in pregnancy leads to high levels of circulating Iodothyronines.
Fall in free T3 & T4 levels, removal of inhibitory feedback hence secrete more TH. Hence free TH levels return to normal but total has increased.
Total and free conc of T4?
Total = 100nmol/L
Free= 20
Total and free conc of T3?
Total= 2nmol Free = 8
Where are thyroid hormones degraded?
Degraded by removal of I
In liver and kidneys.
T4: 7-9 days
T3: 1-2 days
Where is it most common for a failure to occur leading to Thyroid Hormone disease?
Primary Thyroid Gland Failure.
Issue with Negative Feedback
What is Hyperthryoidism?
Over-secretion of T4&T3
Signs & Symptoms of Hyperthryoidism?
Weight loss
Increased appetite
Hyper-reflexive (tremors)
Heat intolerance so sweating
Increased BMR
Osteoporosis so increased turnover of bone and preferential resorption
High T4, low TSH
Skeletal and cardic myopathy hence weakness and breathlessness
Intestinal hyper-mobility hence diarrhoea
EXOPHTHALMOS - starry eyed
PROPTOSIS - eyes bulging out because of fat
Physical and mental hyperactivity
Faster responsiveness of tissues (heart&GI) and regulatory factors (hormones and nt)
Tachycardia
Palpitations
Higher cardiac output
Higher O2 consumption
What disease is associated with Hyperthryoidism?
Graves Disease.
Affects 1%
Mostly affects women
Autoimmune disease as antibodies stimulate T3 & T4 release
Treatment for Hyperthyroidism?
Carbimazole
Inhibits incorporation of iodine to Thy
Inhibits enzyme peroxidase hence prevents
coupling & iodisation of tyrosine on Thy.
Radioactive iodine
Surgery
What is Hypothyroidism?
Under-secretion of thyroid hormones
Signs and Symptoms of Hypothyroidism?
Low T4, high FSH Dry and flaky skins Constipation Weight gain Tiredness and lethargy Mood swings --> depression Cold intolerance Decreased BMR Poor concentration, memory in adults Neuromuscular effects -weakness -muscle cramp -cerebellar ataxia (degeneration) Bradycardia Oedema Deep husky voice Alopecia
Condition occurring in new horns with hypothyroidism?
Cretinism
Severe mental retardation as underdeveloped CNS
Course features
Protruding tongue
Diminishes linear growth
Delayed sexual development
Reversible if treated within few weeks
SCREENING OF ALL NEW BORNS USING ASSAY OF T4 & TSH
Disease associated with Hypothyroidism?
Hashimoto’s disease
Affects 1%
Autoimmune destruction of thyroid follicles, or antibodies that block TSH receptors on follicles
Treatment for Hypothyroidism?
Oral T4 (100-200ug/day) Dose carefully monitored until no signs of hypo or hyper. Once T4 is normal, TSH become normal.
When do goitres occur?
In both hyper and hypothyroidism. It indicates Thyroid failure but does not differentiate. Only blood test for T4 and TSH can differentiate.
