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Metabolism > Thyroid Gland > Flashcards

Thyroid Gland Flashcards

0
Q

What is the histiological structure of the thyroid gland?

A

Follicular cells + cuboidal/columnar epithelia surrounding colloid (which contains thyroglobulin - NOT cytoplasm)

C-cells (darker-staining) produce calcitonin

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1
Q

What is the anatomical location of the thyroid gland?

A

Anterior to the upper trachea (cricoid cartilage) and wrapped around the lower larynx anteriorly

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2
Q

What is the precursor for thyroid hormone synthesis? Outline how T4 & T3 are related.

A

Tyrosine residues on thyroglobulin proteins (therefore fat-soluble)

                                          deiodination (peripheral tissues) Tetra-iodothyronine (T4 = thyroxine) ----------> Tri-iodothyronine (T3)

Note: T4 is the inactive prohormone

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3
Q

What is the hypothalamic-pituitary axis for thyroid hormone synthesis?

A

HYPOTHALAMUS: secretes thyrotrophin-releasing hormone (TRH)

PITUITARY: secretes thyroid-stimulating hormone (TSH)

THYROID: acute effects: synthesis, storage, and secretion of T3/T4
chronic effects: growth and division of follicular cells

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4
Q

What are the functional differences between T3 & T4?

A

T3 more potent/active than T4

T3 has shorter half-life than T4

T3 produced in fewer quantities than T4

T3 has lower binding affinity for TBG than T4

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5
Q

How are T3 & T4 transported in the blood?

A

99% bound to thyronine binding globulin (TBG) (rich in tyrosine)

Some bound to albumin & pre-albumin (TBPA)

1% free (unbound)

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6
Q

What other factors apart from thyroid hormones can affect TBG synthesis and clearance of T3 & T4?

A

Oestrogens, stress, cold, & exercise = stimulates TBG synthesis and clearance of T3 & T4

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7
Q

What is unique about the receptors binding to T3 & T4?

A

On cell membrane AND nuclear membrane

same as with cortisol

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8
Q

Outline the synthesis and release of T3 & T4 in follicular cells.

A

Active transport of iodine into follicular cells (Na+/I- symporter)

Synthesis of tyrosine-rich TBG

H2O2 lyses vesicle containing TBG precursor

Exocytosis of TBG into colloid, and iodination of side chains and coupling to MIT & DIT to form T4 and T3 (STORAGE)

(RELEASE) Endocytosis of TBG-T4 and lysing of vesicle by lysosome

Breakdown produces MIT & DIT which can be deiodinated to form I-

Release of T3 & T4 into bloodstream

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9
Q

How are thyroid hormones inactivated, and where?

A

T3 & T4 degraded by deiodination to T2 in liver and kidney (iodine recycled)

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10
Q

What are the general effects of thyroid hormones?

A
  • increase BMR (e.g. no. & size of mitochondria, increase in oxygen consumption & heat production, increase in nutrient utilisation)
  • stimulates catabolic pathways (lipolysis, glycoloysis, proteolysis, etc.)
  • promotes normal growth & development of tissues (synthesis of specific proteins)
  • increases responsiveness of tissues to sympathetic nervous system (noradrenaline)
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11
Q

What are the specific effects of thyroid hormones?

A

NERVOUS: increased myelination, speed of reflexes, mental activity (alertness, emotional tone, memory)

CARDIOVASCULAR: increased cardiac output, heart rate (directly and by noradrenaline synthesis)

SKIN: increased turnover of proteins & glycoproteins

BONE: increased bone turnover and resorption

GI: increased gut motility

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12
Q

What are the causes, signs, and symptoms of hypothyroidism in the new born? What is the diagnosis and treatment?

A

Cretinism (congenital lack of TSH due to pituitary tumour/iodine deficiency)

  • failure in CNS development —> severe mental retardation
  • decreased muscle & bone development —> diminished linear growth
  • delayed sexual development

Diagnosis:
Measure [T3] & [T4] (newborns screened)

Treatment:
T4

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13
Q

What are the signs and symptoms of hypothyroidism in adults?

A

S&S:

  • cold intolerance
  • decreased perspiration
  • cold, dry hands
  • weight gain
  • bradycardia (low heart rate)
  • decreased cardiac output
  • constipation
  • mood swings/anxiety/depression
  • myxoedema (dry, waxy swelling of skin & subcutaneous tissue)
  • poor concentration & memory
  • dry skin, brittle nails, and hair loss
  • GOITRE
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14
Q

What are some of the causes of hypothyroidism? What is the treatment?

A

Hashimoto’s thyroiditis = autoimmune destruction of thyroid follicles due to production of antibody that blocks the TSH receptor on follicular cells

(Post-partum thyroiditis transient due to modifications to immune system)

Iodine deficiency

Post-sugery/radioactive treatment/anti-thyroid drugs

Congenital

Pituitary tumours (causing lack of TSH)

Treatment: oral T4 (note: dosage adjustment will be necessary until TSH secretion becomes normal)

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15
Q

What can cause a goitre in hypothyroidism?

A

High TSH overcompensating for low T3 & T4 (primary gland failure) causing hypertrophy of follicular cells

OR: iodine deficiency leading to hyperplasia of follicular cells

OR: lymphocyte infiltration (infection/autoimmune)

16
Q

What are the effects of hyperthyroidism in children?

A

Earlier maturation

17
Q

What are the signs and symptoms of hyperthyroidism in adults?

A

S&S:

  • heat intolerance
  • increased perspiration
  • warm, moist hands
  • weight loss
  • increased appetite
  • diarrhoea
  • tachycardia (increased heart rate)
  • increased cardiac output
  • nervousness, irritability, emotional lability
  • hyper-reflexive (tremor in outstretched hands)
  • exophthalmos (proptosis = bulging eyeballs)
  • GOITRE
18
Q

What are the causes of hyperthyroidism?

A
  • Grave’s disease = autoimmune production of antibody that stimulates TSH receptors on follicular cells
  • Toxic multinodular goitre = overproduction of T3 & T4 independent of TSH
  • Solitary toxic adenoma
  • Excessive T3 & T4 therapy/iodine therapy/anti-arrhythmics e.g. anioderone
  • Ectopic thyroid tissue
  • TSH secreting tumours
19
Q

What causes a goitre in hyperthyroidism?

A

Increased production of T3 & T4 despite low TSH (due to negative feedback)

Or lymphocyte infiltration (autoimmune/infection)

Or: tumour

20
Q

What are the treatments for hyperthyroidism?

A
  1. Carbimazole (inhibits incorporation of iodine into TBG)
  2. Radioactive iodine (trapped in thyroid gland —> destroys) —> hypothyroidism
  3. Surgical removal (careful not to remove parathyroid glands)
21
Q

How can the function of the thyroid be screened?

A

Technetium 99m shows areas of high activity in the thyroid when viewed with a gamma camera

22
Q

What is compensated euthyroidism?

A

Borderline hypothyroidism (asymptomatic)

May progress to mild hypothyroidism

23
Q

What is sick euthyroid syndrome?

A

Sick patients often have abnormal thyroid hormone levels due to stress & trauma

Metabolism (20 decks)

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