Thyroid gland

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Thyroid gland thyroid hormones
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Components of the endocrine system
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Thyroid hormones
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Control of thyroid gland

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Hypothalamic releasing hormones affecting
anterior pituitary

• Hypothalamus
• (releasing hormone)
• Thyrotrophin releasing hormone
• (TRH)
• Polypeptide hormone (3aa)

• Anterior Pituitary
• (trophic hormone)
• Thyrotrophin
• Thyroid stimulating hormone
• (TSH)
• Glycoprotein hormone

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Control of thyroid gland
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Actions of TSH on thyroid gland

• Binding of TSH to receptors on the surface of the
  follicle cells
• Acute effects (min-hr)
• Stimulates synthesis, storage secretion of
  T3 T4
• Chronic effects (days)
• Stimulates growth division of follicle
  cells gland enlarges producing a goitre.
• N.B enlarged thyroid can be normal,
  under-active or over-active.

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Synthesis of T3 T4

• Synthesis requires
• iodination of tyrosine.
• coupling 2 tyrosine molecules.
• Tyrosine molecules are part of the polypeptide
  chain of thyroglobulin
• synthesised in follicular cells.
• stored extracellularly in colloid.

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Transport of thyroid hormones
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Mechanism of thyroid hormone action
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General effects of thyroid hormones

• Increase Basal Metabolic Rate
• ? number size of mitochondria.
• ? O2 consumption heat production.
• ? nutrient utilisation.
• Stimulate most metabolic pathways
• Catabolic gt anabolic (lipolysis, glycolysis,
  glycogenolysis, proteolysis).
• Promote normal growth development of tissues
• ? synthesis of specific proteins.
• Increase responsiveness of tissues to sympathetic
  nervous system (noradrenaline) various hormones
  (metabolic reproductive).

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Tissue specific effects of thyroid hormones

• Nervous system development (birth puberty)
  functioning (adults)
• ? myelination of nerve fibres development of
  neurons.
• ? speed of reflexes.
• ? mental activity (alertness, emotional tone,
  memory).
• Cardiovascular system
• ? cardiac output.
• direct effect on heart muscle potentiates
  effect of noradrenaline.
• Skin subcutaneous tissue
• ? turnover of proteins glycoproteins
  (mucopolysaccharides).

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Failure of normal thyroid development.

• Hypothyroidism produces cretinism in the new
  born
• severe mental retardation due to failure in CNS
  development.
• coarse features, protruding tongue.
• diminished linear growth.
• delayed sexual development.
• Reversible if treated within a few weeks.
• ? screen all new born (assay for T4 or TSH)

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Hyperthyroidism hypothyroidism

• Hyperthyroidism
• Signs symptoms relate to
• ? BMR catabolic activity
• ? sympathetic CNS activity
• (GI tract, CNS)
• Direct effects on tissues
• (CVS)

• Hypothyroidism
• ?
• ?
• Direct effects on tissues
• (CVS, subcutaneous)

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Signs symptoms of hyperthyroidism

• Heat intolerance, ? perspiration, warm moist
  hands.
• Weight loss (lipid protein).
• Tachycardia (noticeable heart beat) often
  irregular increased cardiac output.
• Increased bowel movements - increased appetite.
• Nervousness, irritability emotional lability.
• Hyper-reflexive - possible tremor of outstretched
  hands.
• Eye signs - exophthalmos

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Major cause of hyperthyroidism

• Graves disease
• Affects 1 of the population (?/? 10/1).
• Autoimmune disease
• production of antibody that stimulates the TSH
  receptor on follicle cells (LATS).
• Treat with carbimazole inhibits incorporation
  of iodine into thyroglobulin.

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Signs and symptoms of hypothyroidism

• Cold intolerance, decreased perspiration, cold
  dry hands.
• Mild weight gain.
• Bradycardia decreased cardiac output.
• Constipation.
• Mood swings - feeling anxious depressed.
• Poor concentration, poor memory, lack of
  initiative.
• Oedema (myxoedema dry firm waxy swelling of
  skin subcutaneous tissue).
• Dry skin, brittle nails, some hair loss.

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Major cause of hypothyroidism

• Hashimotos disease
• Affects 1 of the population (?/? 10/1).
• Autoimmune disease
• destruction of thyroid follicles.
• production of antibody that blocks the TSH
  receptor on follicle cells.
• Treat with oral T4 (100 200 ?g/day).

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Inactivation of thyroid hormones

• T4 and T3 degraded by removal of Iodine - occurs
  in liver kidney.
• Half-life in plasma T4 7-9 days T3 1-2
  days
• Longer half life of T4 due to greater affinity
  for binding proteins.
• T4 used to treat hypothyroidism easier to
  maintain constant blood concentration.