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Year 2 - Endocrine > Thyroid gland > Flashcards

Thyroid gland Flashcards

1
Q

Vertebral level of thyroid gland

A

C5-T1

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2
Q

What does the thyroid gland produce (3)

A

Thyroid hormones, of 2 forms

  • T3 (triiodothyronine)
  • T4 (thyroxine)

Calcitonin

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3
Q

The hormones T3 (triiodothyronine) and T4 (thyroxine) are produced by the thyroid gland - what do the numbers refer to

A

the number of iodide ions attached to the hormone

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4
Q

Outline the 2 types of cell in the thyroid gland and their arrangement

A

C (clear) cells

Follicular cells - surround hollow follicles by coming together to form the wall of the follicle

Gland is primarily made up of thyroid follicles, surrounded by C cells

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5
Q

Thyroid gland is made up of partly thyroid follicles - what do the centre of these follicles contain

A

Colloid = sticky glycoprotein matrix that stores thyroid hormone and thyroglobulin until needed to be released

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6
Q

What do thyroid follicles store

A

2-3 months supply of thyroid hormones

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7
Q

Function of C (clear) cells - a component of the thyroid gland

A

secrete calcitonin (Regulates Ca2+ - lowers plasma Ca2+)

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8
Q

Functions of follicular cells (4)

A

produces enzymes needed to synthesise thyroid hormones

produces thyroglobulin (protein rich in tyrosine)

surrounds hollow follicles

concentrates iodide from plasma and transports it into the colloid of thyroid follicles

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9
Q

What class of hormones are thyroid hormones

A

amine hormones

-as they’re derived from tyrosine

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10
Q

How do be obtain tyrosine and iodide

A

diet

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11
Q

How do iodide ions in the blood enter follicular cells in the thyroid gland to aid the formation of thyroid hormones (2)

  • transport is coupled to what ion
  • one entered into the follicular cell, where do the iodide ions next get transported to
A

via a Na+/I- transporter (symport)
-iodide needs to be coupled to Na+ to enable the follicular cells to take up iodide against a conc. gradient

then I- is transported across the follicle and out of it into the colloid via a pendrin transporter

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12
Q

Iodide enters the follicular cells from the plasma via a Na+/I- transporter (symport)
- what is the difference between the way Na+ and I- enters the cell

A

Na+ just enters down conc. gradient

I- is transported into cell against a conc. gradient

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13
Q

Outline the steps by which thyroxine (T4) and tri-iodothyronine (T3) are formed

  • wha produces the enzymes and other substances needed for TH synthesis
  • where are the above transported to
  • what does the enzyme produced above catalyse
  • addition of one iodide ion to tyrosine produces
  • addition of a second iodide ion to tyrosine produces
  • T3 is formed by combination of
  • T4 is formed by combination of
A

Follicular cells produce enzymes and thyroglobulin needed to make thyroid hormones

The enzymes and thyroglobulin are packaged into vesicles and exocytosed into the colloid (in the centre of the follicle)

These enzymes catalyse the ADDITION OF IODIDE IONS (that were transported into the colloid via the Na+/I- symporter and pendrin transporter) to tyrosine residues on the THYROGLOBULIN molecule

  • Addition of one iodide ion to tyrosine –> MIT (monoiodotyrosine)
  • Addition of a second iodide ion to MIT –> DIT (diiodotyrosine)

-Then…
MIT + DIT –> triiodothyronine (T3)
or
DIT + DIT –> tetraiodothyronine aka thyroxine (T4)

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14
Q

Where in the thyroid gland are thyroid hormones synthesised and stored

A

Colloid (centre of thyroid follicle)

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15
Q

Since, thyroid hormones are stored in the colloid, how are they secreted into the ECF

  • how do they get into follicular cell from colloid
  • what happens once it’s transported into the follicular cell
  • how does TH get into plasma from follicular cell
A

When stimulated by thyroid stimulating hormone (produced by the AP), portions of the colloid are endocytosed back into the follicular cell in a vesicle

Within the follicular cell, the vesicles then fuse with proteolytic enzymes that cleave the thyroglobulin to release the thyroid hormone

Thyroid hormones are lipid soluble so pass across the membrane into the plasma
- sometimes transport protein (such as MCT transporter) may be involved in moving it into the plasma

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16
Q

Although thyroid hormones are amine hormones derived from tyrosine, why do they not objectively belong to this class

A

because amine hormones are usually hydrophilic/lipophobic

but T3 and T4 are hydrophobic

17
Q

Is most of T3 and T4 circulating in the plasma free or protein bound

A

99.8+ is bound to thyroxine binding globulin

18
Q

Ratio of T3:T4 circulating in the plasma

A
  1. 3nmoles/l : 100nmoles/l
    - so about 50x more total T4
    - most TH circulates in the form of protein bound T4
19
Q

Although there’s 50x more total T4 in the plasma than T3, which is more physiologically active + why

A

T3 because TH receptors inside cells have higher affinity for T3; also there’s more unbound T3 than T4
-90% of TH binding to TH receptors is by T3

20
Q

How is T4 converted into T3

A

T4 is deiodinated to T3 by deiodinase enzymes

- some is deiodinated in the plasma and the rest is deiodinated inside target tissue

21
Q

What external stimuli trigger TRH and subsequent TH release (2)

A

Cold

Exercise

22
Q

What hormones inhibit TH release (2)

A

Somatostatin (GHIH) - inhibits TSH release from AP

Glucocorticoids - inhibit TSH and conversion of T4 to T3

23
Q

Location of T3/T4 receptors

A

In the nucleus of target cells

24
Q

Functions of thyroid hormone (T3) - not T4 has TH receptor has much higher affinity for T3; T4 needs to be converted to T3 to have an effect (6)

A

raises metabolic rate and promotes thermogenesis,

increases gluconeogenesis

net increase in proteolysis

net increase in lipolysis

stimulates GH receptor expression –> further promoting growth, esp LONE BONE GROWTH

promotes brain development in utero

25
Q

How does thyroid hormone raise metabolic rate and promote thermogenesis

A

typically through promoting pointless (FUTILE) cycles of simultaneous catabolism and anabolism

26
Q

Causes of hyperthyroidism (2)

A

Graves disease (autoimmune disease)

Thyroid adenoma - hormone secreting thyroid tumour

27
Q

Pathophysiology of graves disease

A

TSH mimicking antibodies bind to TSH receptors and continually activate the thyroid gland –> overproduction of TH

So strong negative feedback on TRH and TSH

Thyroid gland hypertrophies as well because of overstimulation by these antibodies

28
Q

Effects of hyperthyroidism + subsequent symptoms (4)

A

Increased metabolic rate + heat production –> weight loss/heat intolerance

Increased protein catabolism –> muscle weakness/weight loss

Altered nervous system function –> hyperexcitable reflexes + psychological disturbance

Exaggerated CVS function - increased HR –> increased CO

29
Q

Causes of hypothyroidism (3)

A

Hashimoto’s disease - autoimmune attack of thyroid gland

Deficiency in dietary iodine

Idiopathic

30
Q

Effects of hypothyroidism + subsequent symptoms

A

Decreased metabolic rate and heat production –> weight gain/cold intolerance

Disrupted protein synthesis –> brittle nails/thin skin

Altered nervous system function –> slow speech, hyporeflexia

Reduced CVS function –> slow HR

31
Q

What is a goitre

A

Enlarged thyroid gland

32
Q

Why does the thyroid gland enlarge in hypothyroidism as well as hyperthyroidism

A

TSH secretion is increased because of low levels of thyroid hormones so continually stimulating thyroid gland, but thyroid gland can’t respond to make thyroid hormone because deficient in iodine so TH levels remain low but gland just keeps getting stimulated and enlarges

33
Q

Define euthyroid

A

Normal thyroid function

34
Q

What type of epithelial cells line thyroid follicles

A

Cuboidal

Year 2 - Endocrine (21 decks)

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