Thyroid Disease Flashcards
Location of thyroid gland
In the neck
Anatomy of thyroid gland
Rich blood supply
Left and right lobe
Histology of thyroid gland
Made up of follicles which contain thyroglobulin
Function of Thyroid Hormones
• increase heat production
• increase oxygen consumption
• increase metabolism of proteins, fats and carbohydrates
• promote normal growth
• required for normal CNS function
3 types of thyroid hormone
T4
T3
Reverse
T3
• T3 is metabolically (~ 4x) more active than T4
-Half life is 1.5 days
-can be made from converting t4
T4
Less active Than t3
Half life is 9 days
Can be converted to t3,reduced systemic illness and by drugs eg propanol
Thyroid Hormone Binding Proteins
T4: >99.95% protein bound T3: >99.50% protein bound
Binding Proteins:
• thyroxinebindingglobulin(TBG):~70% • albumin: ~20%
• pre-albumin: ~10%
Concentration of thyroid in the plasma
Thyroxine (T4):
Total T4: 60-150 nmol/L Free T4: 9-26 pmol/L
Tri-iodothyronine (T3):
Total T3: 1.2-2.9 nmol/L Free T3: 3.0-8.8 pmol/L
Thyroid Hormones: Regulation
• negative feedback control of T4 and T3
• free and not total [T4/T3] determine
feedback
• thyroid stimulating hormone (TSH) and thyroid releasing hormone (TRH) involved in feedback
• plasma [TSH] is 0.3-30 mU/L
Thyroid Hormones: Regulation
-hypothalamus release trh thy acts on anterior pituitary and stimulates the release of tsh which acts on the thyroid gland
Thyroid Hormones: Synthesis
• dietary iodide (fish, eggs, milk, salt) absorbed rapidly by small intestine
• around 1/3 of this iodide is concentrated in thyroid gland and 2/3 is excreted by kidneys
• iodide oxidized to iodine in thyroid follicular cells
• iodination of tyrosines on colloid thyroglobulin forms monoiodotyrosine (MIT) and diiodotyrosine (DIT)
• coupling forms T4 (DIT + DIT) and T3 (DIT + MIT)
• thyroid hormones stored attached to colloid thyroglobulin
• thyroglobulin taken up by follicular cells via pinocytosis
• lysosomes fuse with this droplet and proteases digest thyroglobulin to release thyroid hormones
• released thyroid hormones transported in blood bound to thyroxine-binding globulin (TBG)
Inadequate thyroid activity
Hypothyroidism clinically known as myxoedema
Excessive thyroid activity
Hyperthyroidism clinically known as thyrotoxicosis
Incidence of hypothyroidism in the uk
• 2.0% women • 0.2% men
What age does hypothyroidism affect
• mostcommonin50-60yearolds
Causes of hypothyroidism
Primary:
• Hashimoto’sthyroiditis
• post-thyroidectomy
• anti-thyroiddrugs
• congenitalhypothyroidism • iodinedeficiency
Secondary:
• pituitary/hypothalamicdefect
Hypothyroidism Clinical symptoms
• weight gain
• puffy face, hands and feet
• mental / physical slowness
• reduced appetite
• dry skin, hair loss
• deep hoarse voice
• bradycardia
• constipation
• cold intolerance
• menstrual abnormalities
Management of Hypothyroidism
Replacement therapy with T4
• readily available • safe
• inexpensive
Monitoring of therapy with TSH
Incidence of hyperthyroidism in the uk
• ~1.0%women
• ~0.1%men
Causes o hyperthyroidism
Common causes:
• Graves’disease
• toxicnodules
• thyroidhormoneintake
Rare causes:
• ectopicthyroidtissue
• TSHsecretingtumours
Clinical symptoms of hyperthyroidism
• weight loss
• hyperkinesis
• increased sweating
• increased appetite
• tachycardia
• diarrhoea
• heat intolerance
• menstrual abnormalities
Management of hyperthyroidism
Anti-thyroid drugs
• carbimazole Radioiodine therapy
• 131I therapy Thyroidectomy
Thyroid Function Tests
Total T4 and Total T3
• Total T4 is high in hyperthyroidism and low in hypothyroidism
• Total T3 is highly raised in hyperthyroidism but can be normal in hypothyroidism
• Total T4 and total T3 are affected by changes in TBG levels
Altered TBG Levels
increased TBG: increased total T4/T3 increase in TBG occurs with:
• pregnancy
• oral contraceptive intake
• inherited TBG excess
Decreased TBG Levels
-decreased total T4/T3
-decrease in TBG occurs in:
• severe illness
• protein loss eg nephrotic syndrome
• malnutrition
• androgens
• Cushing’s syndrome
Thyroid Function Tests:free t4 and t3
• Free T4 is high in hyperthyroidism and low in hypothyroidism
• Free T3 is highly raised in hyperthyroidism but can be normal in hypothyroidism
• Have largely superseded total T4/T3 measurements
Thyroid Function Tests;tsh
• Plasma TSH is high in hypothyroidism
• TSH is low in hyperthyroidism
• Used as a first-line test in many laboratories
Thyroid function tests:thyroid antibodies
• Measurement useful in assessing autoimmune disease
Thyroid Function Tests: Non-thyroidal illness (sick euthyroid syndrome):
• Thyroid regulation and metabolism disturbed in systemic illness
• Increased conversion of T4 to reverseT3 and not T3 therefore T4 and T3 often low
• TSH may be normal or low
• Wait until patient recovers from illness before testing thyroid function
Thyroid Function Tests: T3 Toxicosis:
• These patients have very high T3 but T4 within reference range
• TSH often very low or undetectable
• Patients often present with clinical hyperthyroidism
Effect of TBG on total
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