Thyroid gland Flashcards
Wha hormones are synthesised in the thyroid?
T3 - tri-iodothyronine
T4 - thyroxine
How are T3 and T4 synthesised? Where does this take place?
Iodination of tyrosine on thyroglobulin
Lumen of thyroid follicles
What regulates thyroid hormone synthesis and secretion?
TRH - thyrotropin releasing hormone
TSH - thyroid stimulating hormone
Plasma iodide
Is there lots of T3 or T4 in the blood? What is their turnover rate?
T3 - small pool, fast turnover rate, intracellular
T4 - large pool in food, slow turnover
Why may T4 be classed as a pro hormone?
Converted to T3 in target cells
T3 much more active than T4
What effect do thyroid hormones have?
Increase metabolism of carbohydrates, fats and proteins
Normal skeletal development and growth
Maturation of the nervous system
Milk production
What hormones do T3 and T4 work in conjunction with?
Insulin
Glucagon
Glucocorticoids
Catecholamines
What can be measured that shows thyroid hormone action?
Increased oxygen consumption
Heat production
Increased basal metabolic
What effect does hyperthyroidism (and thyroid hormone drugs) have on the heart?
Increases heart rate
Increased arrhythmias
How do thyroid hormones influence growth and developement?
Direct action on cells
Indirectly by influencing growth hormone
Which animals are hypothyroidism and hyperthyroidism particularly common?
Hyperthyroidism - old cats
Hypothyroidism - dogs
What is goitre? What causes it?
Swelling of thyroid gland
Dietary iodine deficiency
What does a dietary deficiency of iodine or goitrogens cause?
Reduced thyroid hormone synthesis
Rise in plasma TRH
Increased gland size (goitre)
How is hyperthyroidism diagnosed?
History, clinical signs
Total thyroxin
(Cheap, easily available)
How is hyperthyroidism managed?
Diets restricted in iodine Surgery Drug treatment (thioureylenes and radioactive iodide)
What is a danger when operating on the thyroid gland?
Damaging parathyroid gland
How can hypothyroidism be diagnosed?
History, clinical signs
Total blood thyroxine (not always reliable - false positives)
Why is total blood thyroxine often not reliable to diagnose hypothyroidism?
Euthyroid sick syndrome
= total blood thyroxine often reduced as a physiological response to other illnesses
Drugs can also reduce tT4
What can be used with tT4 in dogs to diagnose hypothyroidism?
Elevated TSH - loss of negative feedback
TSH alone is not useful, but useful with tT4
How can free T4 (fT4) be used to differentiate euthyroid sick syndrome from genuine hypothyroidism?
Free T4 rarely suppressed by other illnesses unlesss severe
High specificity
What 2 drugs can be used for hyperthyroidism?
Radioiodine
Thioureylenes
What 2 drugs can be used for hypothyroidism?
Levothyroxine
Liothyronine
How does radioiodine work? How long does it take to normal thyroid levels?
Rapidly uptaken by thyroid Damages cells by emitting short range beta radiation
1-2 weeks (half life 8 days)
How is radioiodine administered? What precautions should be taken?
Injection
Treated animals need hospitalisation and isolation for at least 2 wks, need careful handling
Radioiodine injections are very expensive!!! Why may some people still use it rather than surgery?
Less risk of damage to parathyroid
What are the adverse effects and contraindications of radio iodine?
Hypothyroidism
Pregnancy
Give 2 examples of thioureylenes
Carbimazole
Thiamazole
How do thiouryelenes work?
Metabolised to methimazaole which inhibits synthesis of thyroid hormones
How are thioureylenes administered? What are the adverse effects?
Orally - once a day
Rare - can decrease GFR and unmask hidden renal failure (CKD)
How do levothyroxine and liothyronine work? How do they travel in the blood?
Thyroid receptor agonists
Highly protein bound - only free hormone metabolically active
How are levothyroxine and liothyronine administered? What are the adverse effects?
Orally
Hyperthyroidism
Enhances actions of catecholamines and sympathomimetics
What are the symptoms of hyperthyroidism?
Tachycardia Excitability Excessive panting Weight loss Increased appetite
What are the symptoms of hypothyroidism in the dog?
Excess shedding/thinning of fur
Weight gain
Reduced activity
