thyroid

Question 1

function of thyroid hormones

Answer 1

• normal growth and development
• maintain metabolic activity and oxygen requirements especially brain
• regulate lipid and carbohydrate metabolism and thus body weight

control of thyroid hormone is by HPA

Question 2

thyroid anatomy and supply

Answer 2

• consists of 2 lobes connected by the isthmus
• deep to sternohyoid muscle
• anterior to recurrent laryngeal nerve and tracheal cartilage ring 2 & 3
• has ANS innervation
• supplied by superior and inferior thyroid artery

Question 3

thyroglobulin

Answer 3

• colloid is made up of thyroglobulin
• Tg is a glycoprotein that stores thyroid hormone
• Tg is a cancer marker (normally circulates in blood in low levels)

Question 4

follicular epithelium

Answer 4

inactive thyroid = cuboidal
active thyroid = FSH makes cells columnar

produce Tg
closely associated with capillaries

Question 5

parafollicular cells

Answer 5

• between follicles

- produce calcitonin hormone

Question 6

thyroiditis

Answer 6

• inflammation of thyroid
• increased thyroid hormone in thyroid releases into the blood circulation
• leads to high thyroid function = high Tg in blood

Question 7

factitious thyrotoxicosis

Answer 7

when patients take thyroid hormones deliberately and subsequently make themselves thyrotoxic

• patients present with weight loss and all typical features of an overactive thyroid
• however, Tg is very low b/c thyroid has gone to sleep

Question 8

iron deficiency leads to

Answer 8

• compensatory enlargement of thyroid (endemic goitre)

- in pregnancy may lead to fetal thyroid not developing properly and irreversible damage to developing CNS

Question 9

thyroid hormone synthesis

Answer 9

• 90% thyroid hormone produced in thyroid gland is T4
• most T3 bound to proteins such as thyroid binding globulin and albumin acts as a buffer against acute changes in T4 production and for iodine recycling

Question 10

carbimazole

Answer 10

drug given to patients with an overactive thyroid hormone

• inhibits peroxidase enzyme, thus reducing iodine uptake into thyroid and decrease thyroid hormone production

Question 11

thyroid hormone control mechanisms

Answer 11

1. hypothalamus release TRH
2. pituitary release TSH
3. stimulates thyroid gland
4. thyroid produces thyroid hormones (T4 & T3)
   
   • T3 also produced locally at target organs

• T3 & T4 has a negative feedback at hypothalamus and pituitary
• TSH has negative feedback on pituitary

Question 12

thyrotoxicosis hormones and signs/symptoms

Answer 12

overactive thyroid with high T4/T3 and low TSH

symptoms

• nervousness, increased sweating
• weight loss
• tachycardia
• weakness and tiredness

signs

• skin changes
• tremor
• eye signs
• goitre
• tachycardia

Question 13

causes of thyrotoxicosis

Answer 13

1. graves disease
   = autoimmune production of an antibody that stimulates TSH receptors
2. multi-nodular goitre
   = hyperfunctioning regions of thyroid gland, not suppressed by circulating thyroid hormone
3. thyroiditis
4. drugs
5. toxic nodule

Question 14

metabolism of thyroid hormone

Answer 14

when T4 is released and reaches target organs (esp liver & kidney) it locally turns into T3. T3 is the form that has metabolic activity in the periphery

thyroid hormone is metabolised, broken down and excreted by liver and kidneys

Question 15

high T4, low TSH

Answer 15

if high T4 appropriate feedback = less TSH which normally inhibit thyroid with decreased T4

therefore, overactive thyroid causing increased T4 release, with normal pituitary

• in some conditions of pregnancy HCG can be high cause overactivity of thyroid

Question 16

low T4, high TSH

Answer 16

if low T4 appropriate feedback = more TSH which normally stimulates thyroid with more T4

therefore underactive thyroid causing decreased T4 release, with normal pituitary

Question 17

thyroid hormone synthesis: iodide ‘trapping’

Answer 17

• iodide conc in plasma is extremely low
• iodide is trapped by a sodium-iodide symporter (NIS)
• active transport of 2 Na+ ions results in the entry of 1 iodide molecule against its conc gradient

Question 18

where is the sodium-iodide symporter found?

Answer 18

• breast
• gastric mucosa
• ciliary body eye
• salivary glands
• differentiated thyroid cancer cells

Question 19

iodide oxidation

Answer 19

oxidation of iodide is catalysed by thyroid peroxidase (TPO)

in the presence of H2O2, TPO iodinates tyrosine residues in Tg

TPO is inhibited by carbimazole

Question 20

storage and release of thyroid hormone

Answer 20

• large store of thyroid hormone (T4/T3) incorporated into Tg (~50 day supply)
• thyroid hormone release involves endocytosis of Tg (colloid) from follicular space
• endosomes fuse with lysosomes
• degradation of Tg –> T4 or T3 and released
• thyroid gland only place where T4 synthesied but T3 also from peripheral deiodination of T4
• T4 & T3 metabolised by many organs esp liver/kidneys

Question 21

TSH stimulates what aspects of thyroid hormone synthesis

Answer 21

• increase iodide into follicular lumen
• increased BF
• increased Tg, TPO and H202
• increased endocytosis and degradation of Tg
• increased release of T4

Question 22

peripheral metabolism of thyroid hormones

Answer 22

T4 is the main thyroid hormone in plasma

T3 is also released from thyroid
-80% is generated by peripheral metabolism from T4 by removal of a single iodine atom

Question 23

primary hypothyroidism

Answer 23

=hashimotos disease

high TSH, low T4, +ve TPO antibodies

• adult onset slow
• can affect all organ systems
• decrease in energy metabolism e.g. low basal metabolic rate +/- slightly low body temperature
• decreased protein synthesis