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Functions and disorders of the thyroid gland, Thyroiditis (3%) - Coggle…
Functions and disorders of the thyroid gland
Thyroid
Has very good blood supply
Sits wrapped around the trachea below the thyroid cartilage (adam's apple)
Has parathyroid glands beside it
Has a left and right lobe
Has an isthmus in the middle
Thyroid hormones
Thyroid hormone actions
Thermogenesis in brown adipose tissue
Activation of mental processes
Basal metabolic rate
Growth
On cells
hormones brought into cells via active transport
T3 regulates a transcription factor
Nuclear T3 receptors
it causes the generation of proteins which modify metabolism function and growth
T4 is transformed into T3 and rT3
This is mediated by the deiodinase enzyme
D1 and D2 transforms it to T3
D1 and D3 transforms it to rT3
in many cases the produced T3 is exported back into the blood rather than acting on TF in the cells
Thyroxine (T4)
Triiodothyronine (T3)
Its inactive form is called reverse triiodothyronine
Both formed from iodinated tyrosine residues
Synthesis
Thyroid epithelial cells trap iodine
An Na-Iodine transporter pump pushes it into cells, specifically the colloid
The cells then produce thyroglobulin
this is full of tyrosine residues
These residues are iodinated to form thyroid hormones within thyroglobulin. This is mediated by TPO and H2O2
The hormones formed are T3, T4, DIT and MIT
The thyroglobulin residues are then taken out of the colloid via colloid reabsorption
it undergoes proteolysis releasing all of the thyroid hormones which are then secreted
the whole process is sped up by TSH as it causes an influx of calcium leading to the release of thyroglobulin containing vesicles
DIT
Diiodothyronine
MIT
monoiodothyronine
Transport
Mainly thyroid hormones circulate in the blood bound to proteins (95.95% bound)
70% bound to thyroxine binding globulin
20% bound to transthyretin
10% bound to albumin
Bound hormones are inactive and free hormones are active
thyroid follicle
Thyroid epithelial cells sit in a circle around the Colloid
Between these cells and the colloid is where thyroid hormones are formed
Closely associated with C-cells
these cells secrete calcitonin which affects calcium balance
Hypothalamus-pituitary-thyroid system
Hypothalamus releases thyrotrophin releasing hormone (TRH)
anterior pituitary generates thyroid stimulating hormone in response (TSH)
TSH acts on the thyroid causing the release of T4 and T4
This then acts on target tissues and reduces the level of TSH hormone
More T4 is produced
Iodine deficiency
thyroid needs iodine to work
Thyroid gland increases in size to capture as much iodine as possible when deficient
Clinical aspects of thyroid dysfunction
2-5% of people have thyroid dysfunction
Generally thyroid dysfunction fits into 2 categories
Hyper/Hypothyroidism
hyperthyroidism
Clinical features
Weight loss
Increased apetite
Palpitations
Fatigue
Heat intolerance
Anxiety
Sweating
Tremor
Muscle weakness
Causes
Graves disease (76%)
Caused by stimulating antibodies to the TSH receptor
Immune system attacks the TSH receptor aggravating it and stimulating the production of T3 and T4 causing the thyroid gland to grow
Using radioactive imaging you can observe the thyroid activity and determine if the whole gland is hyperactive - if so diagnose
Symptoms of ophalmopathy
eyelid retraction and lag
Proptosis (Tissue behind the eye pushes outwards)
Redness of eyes
Gritty sensation
Swelling
Blurred vision
Caused by TSH action in tandem with cytokine action on orbital fibroblasts causes them to activate
Causes production of glycosaminoglycans and adipose tissue
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Treatment
Carbimazole for 18 months
Inhibits thyroid peroxidase enzyme stopping iodinated tyrosine residues
May decrease TSH receptor antibodies
Propylthiouracil can be used instead - especially in pregnancy
Radioiodine
High risk of hypothyroidism within 1 year of treatment
May worsen opthalmopathy
Sub-total or total thyroidectomy
Beta-blockers can be useful for symptomatic control
Steroids may be useful for treating active ophthalmopathy
Selenium supplementation can reduce inflammation
Multinodular goitre (14%)
At severe stage:
Goitre is nodular
T3 and T4 are elevated
TSH is undetectable
Generally occurs in individuals over 55
Radioactive images show patches of hyperactivity in the gland
Treatment
Radioiodine usually
Lower risk of hypothyroidism in these patients than those with graves' disease
Surgery
This is for individuals with tracheal compression/deviation
single toxic nodule (5%)
Other causes (2%)
Medicine
Hypothyroidism
Clinical features
Weight gain
Triedness
Cold intolerance
Muscle stiffness
Constipation
Hyperlipidaemia
Dry hair and skin
Common causes
hashimoto's thyroiditis
antibodies attack the thyroid and make it underactive
Permanent
Tendency to run in families
Iatrogenic causes (result of treatment)
post surgery or radioactive iodine treatment
Spontaneously atrophies gland
Temporary thyroiditis
Rare causes
hypopituitarism
Congenital causes
iodine deficiency
Drug-induced cases
E.g., lithium, amiodarone
Treatment
Replace T4
Normalises TSH and T4
Return the patient to a clinically euthyroid state
small number of patients may require combined T3 and 4
Thyroid enlargement
Thyroiditis (3%)
Viral
Postpartum
Radioactive imaging shows no sites of high activity
Generally you don't need to treat it