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Endocrinology > Thyroid Gland > Flashcards

Thyroid Gland Flashcards

1
Q

Where is the thyroid gland located?

A
  • Located in the neck
  • Below the thyroid cartialge
  • Moves up when you swallow
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2
Q

What is the morphology of the thyroid gland?

A
  • Butterfly shaped gland
  • 2 lobes
  • Central portion = Isthmus
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3
Q

What cells and other components is the thyroid gland made of?

A
  • Structures called follicles (spheres)
  • Follicular cells
  • Middle portion is colloid (thick mucus like ECF)
  • Parafollicular cells : release calcitonin for calcium metabolism
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4
Q

What structures are in close proximity to the thyroid gland?

A
  • Parathyroid glands embedded in thyroid (important when doing surgery to remove thyroid gland to keep parathyroid gland - calcium metabolism)
  • Left recurrent laryngeal nerve runs close (supplies vocal cords & controls voice)
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5
Q

What is the embryology of the thyroid?

A
  • Midline outpouching from floor of pharynx (originates from base of tongue)
  • Development of thyroglossal duct (runs all the way down neck)
  • Divides into 2 lobes
  • Duct disappears leaving foramen caecum
  • Final position by week 7 ; thyroid gland deveops
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6
Q

What can go wrong in thryoid development?

A
  • No development of gland (throid agenesis)
  • Process stopping along any point in thryogloosal duct, doesnt reach final position
  • Lump in throat, surgeon removes, could remove only source of thyroid production and render them hypothyroid
  • Lingual thyroid
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7
Q

What is the histology of the thyroid?

A
  • Round structures are follicles
  • Purple dots are nuclei of each follicular cell
  • Red blobs are capillaries - blood supply of thryoud
  • Pink - Colloid
  • White patch on follicluar cells - site of thyroid hormone production
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8
Q

What does the binding TSH to TSH receptor lead to?

A
  • Activates sodium iodide transporter which allows iodide ions to enter follicular cell and then enter colloid, iodide oxidised in iodination
  • Activates production of pro-hormone thyroid globulin which enters colloid
  • Activates enzyme called thyroperoxidase TPO. TPO enters colloid and llong with hydrogen peroxide catalyses iodination reactions
  • Catalyses reaction called coupling reaction
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9
Q

How does the coupling reaction that produces T3 & T4 occur?

A
  • Thyroglobulin is pro-hormone made of amino acids including tyrosine (aromatic amino acid)
  • Add iodine to tyrosine = 3-Monoiodotyrosine
  • 2 Iodine to tyrosine = 3,5-Diiodotyrosine
  • 3-Mono + 3,5-Diiodo = 3,5,3’-Tri-iodothyronine (T3)
  • 2 3,5-Diiodo = 3,5,3’,5’-Tetra-Iodothyroxine (T4, Thyroxine)
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10
Q

What is the deiodination process that leads to T3?

A
  • Prohormone thyroglobulin (series of amino acids) with tyrosine residue (100 maybe but 20 can be iodinated)
  • Deiodinate T4 gives T3
  • Position of iodine molecule important, wrong person gives inactive form (reverse T3)
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11
Q

What is the proportion of thyroid hormones T3 & T4?

A
  • Healthy adult thyroid gland secretes T3 & T4
  • T4 prohormone converted by DEIODINASE ENZYME into more active metabolite TRIIODOTHYRONINE (T3)
  • Circulating T3: 80% from deiodination of T4 , 20% from direct thyroidal secretion
  • T3 provides amost all the thyroid hormone activity in target cells
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12
Q

How are thyroid hormones T3 & T4 transported in blood?

A
  • Mostly bound to plasma proteins:
    a) Thyroid-binding globulin : TBG (70%-80%)
    b) Albumin (10%-15%)
    c) Prealbumin (aka transthyretin)
  • Only 0.05% T4 and 0.5% T3 is unbound (bioactive components)
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13
Q

How do thyroid hormones have an effect on gene expression?

A
  • T3 & T4 get to target cells and enter cell via relevant receptor
  • T4 deiodinated by deiodinase enzyme (present in all target cells) to T3
  • T3 enters nucleus via nuclear receptor & has effect on gene expression
  • Target cell carries out task
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14
Q

What affect does thyroid hormone have on foetuses and what happens when they don’t have enough?

A
  • Essential for fetal growth & development, in particular CNS
  • Untreated congenital hypothyroidism : cretinism
  • TSH measured in new-born infants heel prick test

(normally get thyroid hormone from mother)

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15
Q

What does thyroid hormone do?

A
  • Increases basal metabolic rate
  • Protein, carbohydrate & fat metabolism
  • Potentiate actions of catecholamines (e,g, tachycardia, lipolysis)
  • Effects on the GI, CNS, Reproductive systems
  • Thyroid hormones binds nuclear receptor -> synthesis of new proteins
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16
Q

How is the production of thyroid hormone controlled?

A
  • Release of TRH (thyrotropin releasing hormone) from hypothalamus
  • Enters anterior pituitary passing through circulation
  • Stimuates thyrotropes cells to release TSH
  • TSH released into bloodstream arrives at thryoid and results in production of thyroid hormone (T3 & T4)
  • Negative feedback : T3 & T4 produced sends signals to pituitary and hypothalamic level to switch off production of TRH & TSH
  • Somatostatin negatively inhibits anterior pituitary releasing TSH
  • Presence of iodide : too much transiently inhibits production of T3 & T4 (Wolff-CHaikoff effect)
17
Q

What causes primary hypothyroidism? What happens because of it?

A
  • Normally due to autoimmune damage to thyroid
  • Thyroxine levels decline (T4)
  • TSH levels climb (to try make up T4 levels)
18
Q

What are the common forms of autoimmune thyroid diseases associated with hypothyroidism?

A
  • Hashimoto’s thyroiditis
  • Graves’ disease
  • Presenece of autoimmune disease increases risk of others
19
Q

What are the symptoms/signs of hypothyroidism?

A
  • Fatigue, memory impairment, depression
  • Shaggy hair/ hair loss, Cold intolerance

Swollen face, swelling of eye socket, rough/deep/hoarse voice

  • Enlarged thyroid gland
  • Weight gain/constipation
  • Slowed heart rate(bradycardia), weakness
  • Dry, rough skin, paresthesia, muscle cramps
  • Diminished potency and fertility, low sexual desire, cycle disorders
  • Eventual myxoedema coma
20
Q

What drug can be used to treat hypothyroidism?

A
  • Levothyroxine
  • Give thryoxine (T4) in tablet form
  • Now T4 in target organ, deiodinase enzyme deiodinates into T3
  • Has effect on gene expression
  • Blood test to measure TSH & T4
  • TSH high when they have hypothyroidism
  • Most commonly administered orally - starting dose 100 micrograms
21
Q

Can you give Levothryoxine to someone with hyperthyroidism?

A
  • In some cases you will
  • Give drug to completeley stop making own thyroid hormone
  • Antithyroid drug
  • Give back physiological amount of thyroxine
  • Blocking and replacement regimen
  • Not common to give though
22
Q

What are potential complications of levothyroxine?

A
  • Minor: Weight loss/ headaches

Major: Heart attack/ Rapid HR

23
Q

Why is T3 (Liothyronine) not prescribed often?

A
  • Not a huge amount of benefit over T4
  • Patients have normal deiodinase enzyme so can make T3 from T4 drug
  • Needs to be given more than once a day (half life)
  • Expensive
  • Some take combination of T3 & T4 ( can overdo it and symptoms of palpiations, tremor, anxiety)
  • Often combination treatment suppresses TSH
24
Q

What is hyperthyroidism? What happens because of it?

A
  • Thyroid makes too much thyroxine
  • Thyroxine levels rise
  • TSH levels drop (stops anterior pituitary producing TSH)
25
Q

What are some causes of hyperthyroidism?

A
  • Graves’ disease- whole gland smoothly enlarged and whole gland overactive
  • Toxic multinodular goitre (multiple nodules develop within thyroid gand all over produce thyroid hormone)
  • Solitary toxic nodule (single nodule over produces thyroid hormone)
26
Q

What are the signs/symptoms of hyperthyroidism?

A
  • Nervousness, irritability, insomnia, depression
  • Broken hair, hair loss
  • Weight loss, strong feeling of hunger, diarrhoea
  • Enlarged thyroid gland
  • Fragile fingernails, shaky hands
  • Increased heart rate, arrhythmia, high blood pressure
  • Warm, moist skin, increased body temp (feel hot in cold weather), muscle cramps, weakness, cycle disorders
27
Q

What is Graves’ disease?

A
  • Autoimmune disease
  • Antibodies bind to and stimulate TSH receptor in thyroid - smooth goitre
  • Continues to stimulate
  • Thyroid grows diffusely (antibody not specific to one area of thyroid- not nodules that is not autoimmune)
  • Also, other antibodies bind to muscles behind eye - Exophthalmos
  • Other antibodies stimulate growth of soft tissue of shins (pretibial myxoedema)

Endocrinology (5 decks)

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