Thyroid Disease

Question 1

Location of thyroid gland

Answer 1

In the neck

Question 2

Anatomy of thyroid gland

Answer 2

Rich blood supply
Left and right lobe

Question 3

Histology of thyroid gland

Answer 3

Made up of follicles which contain thyroglobulin

Question 4

Function of Thyroid Hormones

Answer 4

• increase heat production
• increase oxygen consumption
• increase metabolism of proteins, fats and carbohydrates
• promote normal growth
• required for normal CNS function

Question 5

3 types of thyroid hormone

Answer 5

T4
T3
Reverse

Question 6

T3

Answer 6

• T3 is metabolically (~ 4x) more active than T4
-Half life is 1.5 days
-can be made from converting t4

Question 7

T4

Answer 7

Less active Than t3
Half life is 9 days
Can be converted to t3,reduced systemic illness and by drugs eg propanol

Question 8

Thyroid Hormone Binding Proteins

Answer 8

T4: >99.95% protein bound T3: >99.50% protein bound
Binding Proteins:
• thyroxinebindingglobulin(TBG):~70% • albumin: ~20%
• pre-albumin: ~10%

Question 9

Concentration of thyroid in the plasma

Answer 9

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

Question 10

Thyroid Hormones: Regulation

Answer 10

• 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

Question 11

Thyroid Hormones: Regulation

Answer 11

-hypothalamus release trh thy acts on anterior pituitary and stimulates the release of tsh which acts on the thyroid gland

Question 12

Thyroid Hormones: Synthesis

Answer 12

• 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)

Question 13

Inadequate thyroid activity

Answer 13

Hypothyroidism clinically known as myxoedema

Question 14

Excessive thyroid activity

Answer 14

Hyperthyroidism clinically known as thyrotoxicosis

Question 15

Incidence of hypothyroidism in the uk

Answer 15

• 2.0% women • 0.2% men

Question 16

What age does hypothyroidism affect

Answer 16

• mostcommonin50-60yearolds

Question 17

Causes of hypothyroidism

Answer 17

Primary:
• Hashimoto’sthyroiditis
• post-thyroidectomy
• anti-thyroiddrugs
• congenitalhypothyroidism • iodinedeficiency
Secondary:
• pituitary/hypothalamicdefect

Question 18

Hypothyroidism Clinical symptoms

Answer 18

• 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

Question 19

Management of Hypothyroidism

Answer 19

Replacement therapy with T4
• readily available • safe
• inexpensive
Monitoring of therapy with TSH

Question 20

Incidence of hyperthyroidism in the uk

Answer 20

• ~1.0%women
• ~0.1%men

Question 21

Causes o hyperthyroidism

Answer 21

Common causes:
• Graves’disease
• toxicnodules
• thyroidhormoneintake
Rare causes:
• ectopicthyroidtissue
• TSHsecretingtumours

Question 22

Clinical symptoms of hyperthyroidism

Answer 22

• weight loss
• hyperkinesis
• increased sweating
• increased appetite
• tachycardia
• diarrhoea
• heat intolerance
• menstrual abnormalities

Question 23

Management of hyperthyroidism

Answer 23

Anti-thyroid drugs
• carbimazole Radioiodine therapy
• 131I therapy Thyroidectomy

Question 24

Thyroid Function Tests

Answer 24

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

Question 25

Altered TBG Levels

Answer 25

increased TBG: increased total T4/T3 increase in TBG occurs with: • pregnancy • oral contraceptive intake • inherited TBG excess

Question 26

Decreased TBG Levels

Answer 26

-decreased total T4/T3 -decrease in TBG occurs in: • severe illness • protein loss eg nephrotic syndrome • malnutrition • androgens • Cushing's syndrome

Question 27

Thyroid Function Tests:free t4 and t3

Answer 27

• 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

Question 28

Thyroid Function Tests;tsh

Answer 28

• Plasma TSH is high in hypothyroidism • TSH is low in hyperthyroidism • Used as a first-line test in many laboratories

Question 29

Thyroid function tests:thyroid antibodies

Answer 29

• Measurement useful in assessing autoimmune disease

Question 30

Thyroid Function Tests: Non-thyroidal illness (sick euthyroid syndrome):

Answer 30

• 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

Question 31

Thyroid Function Tests: T3 Toxicosis:

Answer 31

• These patients have very high T3 but T4 within reference range • TSH often very low or undetectable • Patients often present with clinical hyperthyroidism

Question 32

Effect of TBG on total

Answer 32

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