hypothyroidism Flashcards
Draw a diagram to show the hypothalamo-pituitary-thyroid axis.
How can primary hypothyroidism be diagnosed?
High TSH
What is the role of TSH?
Control uptake of Iodide.
Control enzyme that catalyses thyroxine formation.
Diagram to outline thyroxine synthesis.
Diagram outlining thyroid axis, thyroxine production, thyroxine effects.
What is the most common cause of primary hypothyroidism (myxoedema)?
autoimmune damage to the thyroid.
Outline the pathology of primary hypothyroidism.
autoimmine damage to thyroid.
Thyroxine levels decline.
TSH levels rise.
Give some symptoms and signs of primary hypothyroidism.
Deepening voice
Depression and tiredness
Cold intolerance
Weight gain and reduced appetite
Constipation.
Bradycardia.
Eventual myxoedema coma.
How do T4 and T3 relate to each other?
T4 is a prohormone. Deiodinase enzyme converts T4 –> T3. T3 provides almost all thyroid hormone activity in target cells.
What are the origins of circulating T3.
80% deiodination of T4.
20% direct thyroidal secretion.
Outline the method of action of T3.
T3/T4 enter cell. T4 –> T3 (deiodinase).
T3 binds to retinoid x receptor and thyroid hormone receptor (hexodimer).
This grioup binds to Thyroid response element in DNA which modulates gene expression.
How is primary hypothyroidism treated?
Thyroxine replacement (also called thyroxine sodium or levothyroxine sodium). T3 also given less commonly.
List some causes of hypothyroidism.
Autoimmune, post-thyroidectomy, post-radioacrive iodine intake.
How is thyroxine administered.
Normally orally.
How is thyroxine dose for primary hypothyroidism controlled?
TSH used for guidance of dosage - aim to keep within reference range.
What is secondary hypothyroidism?
Healthy thyroid - lack of TSH to stimulate it
What are some causes of secondary hypothyroidism?
Pituitary tumour, post-pituitary surgery, radiotherapy.
How is T4 dose controlled in secondary hypothyroidism?
Aim for free T4 levels in middle of reference range.
Why might you give T3 instead of T4.
Treatment for myxoedema coma - faster acting + can be asdministered intravenously.
What are the pros and cons of combination T3/T4 treatment?
Some have reported that combination improves well being (limited evidence)
However, can lead to palpitations, tremor, anziety and combination can lead to suppression of TSH.
What is the difference in half-life between T4 and T3.
T4 - 6 days
T3 - 2.5 days
What proportion of T3 and T4 is free?
0.03% (99.97% bound to PP).
What is the primary binding protein for plasma T4 and T3?
Thyroxine Binding Globulin (TBG)
How might TBG levels change and why?
Increase - in pregnancy or on prolonged oestrogen/phenothiazine treatment.
Decrease - malnutrition, liver dieases.
Certain drugs can compete for binding sites (e.g. phenytoin/salicylates).
