Thyroid Disorders

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Thyroid Disorders
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Thyroid Anatomy

• Macroscopically
• The thyroid has two lobes joined by an isthmus
• Embedded into the upper and lower poles of both
  lobes are the parathyroid glands.

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Thyroid Structure

• Microscopically
• The gland is formed of follicles, which are
  rings of thyroid cells enclosing a colloid which
  contains the stored hormones and thyroglobulin
  the precursor

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Thyroid Hormone Formation

• Essential requirements are the protein
  Thyroglobulin, Iodine and enzymes which join the
  two together
• The hormones are composed of tyrosine ( a
  subcomponent of thyroglobulin) and 1, 2, 3 or 4
  Iodine molecules,
• The active hormones are the tri form tri-iodo
  thyronine and thyroxine, which has 4 Iodine
  molecules

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Control of Thyroid hormone production

• Thyroid releasing hormone (TRH) from the
  Hypothalamus causes the pituitary to release
  Thyroid Stimulating Hormone (TSH)
• There is a negative feedback loop between the
  Thyroid hormones and these control hormones
• Iodine is also required

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Action of Thyroid Hormones

• Increase sweating
• Increase cardiac output
• Increase pulse pressure ( higher syst. and lower
  dias.)due to an increase in the stroke volume and
  a reduction in peripheral vascular resistance
• Increased utilization of CHO, Protein and Fat
• Increased excitation of Nervous system
• Plus Thyroid hormones are essential for the
  growth and development of the Skeleton, Teeth,
  Epidermis and CNS

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Thyroid Disorders

• Common disorders are
• Goitre an enlarged thyroid
• Hypothyroidism symptoms of reduced levels of
  thyroid hormones
• Hyperthyroidism symptoms of increased levels of
  thyroid hormones
• Less common are thyroid tumours

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Terms and their definitions

• Goitre enlarged thyroid (does not refer to the
  function of the thyroid)
• Toxic increased thyroid hormone output
• Nodule palpable lump in thyroid

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Goitres

• Can be either simple or toxic
• Diffuse or Multinodular

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• Simple (non-toxic)Goitre
• thyroid is enlarged ( either diffusely or in a
  multinodular form) but there is no excess thyroid
  hormone production
• Aetiology
• Inadequate iodine,
• Excessive amounts of goitrogens
• Pregnancy ( higher need for Iodine)
• Drugs

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Clinical Features of Simple Goitre

• Enlarged thyroid which is soft and symmetrical
• No changes in any of the hormones euthyroid
• Usually no treatment is required and the goitre
  resolves

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Simple Multinodular Goitre

• A gradually enlarging thyroid which develops
  localised areas of hyperplasia resulting in
  palpable nodules
• The gland can become large enough to compress
  nearby structures but it does not usually produce
  excessive levels of hormones
• Unless causing pressure problems or early
  hyperthyroidism the thyroid is merely reviewed
  annually

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Multinodular goitre
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Hypothyroidism

• Most common thyroid disorder
• Is the clinical syndrome that results form a
  deficiency of thyroid hormones
• It can develop in utero (cretinism) or as an
  adult
• If It occurs in the latter situation it can
  result in deposition of glycosaminoglycans (GAGS)
  and thus produce myxedema

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Pre tibial Myxedema
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Causes of Hypothyroidism

• Primary failure of thyroid gland to produce
  thyroid hormone. Can be caused by,
• Hashimotos thryoiditis (/- goitre)
• End stage Graves disease
• Surgical removal of thyroid or radioactive
  ablation of thyroid as treatment of Graves
• Secondary failure of pituitary to produce TSH
• Can also be tertiary

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Hashimotos Thyroiditis

• Most common cause of hypothyroidism and also
  causes a goitre
• Aetiology
• Cause by an auto-immune reaction to thyroid but
  mechanism is still unclear
• There are auto-antibodies to thyroglobulin, etc
  but their levels do not correlate well with the
  severity of the disease

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Hashimotos contd

• Pathology
• The thyroid undergoes infiltration by lymphocytes
  and fibrosis follows resulting in an initially
  enlarged gland shrinking

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Hashimotos thyroiditis
Lymphoid follicles at right and centre
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Clinical Features of Hypothyroidism

• There is usually a goitre and variable levels of
  thyroid hormones
• Some are due to reduced levels of thyroid
  hormones eg slowness, cold intolerance
• Others are due to a build up of GAGS eg
  non-pitting oedema of hands, eyes

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Complications

• 5 develop thyroid cancer
• As it is usually a disease of the elderly it can
  co-exist with CAD so treatment can unmask and
  stress the previously protected coronary vessels

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Thyrotoxicosis

• Definition
• Clinical features of excess thyroid hormone

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Aetiology of Thyrotoxicosis

• Graves Disease, which is associated with
  autoantibodies which act to stimulate TSH
  receptors and thus cause the production of
  greater amounts of thyroid hormone
• A nodule in a multinodular goitre which produces
  excess thyroid hormone ( ie becomes toxic)
• Toxic adenoma

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Clinical Features of Thyrotoxicosis

• General heat intolerance, warm moist skin, LOW
  in spite of increased appetite
• GIT malabsorption and diarrhoea
• CVS palpitations, tachycardia
• Neuromuscular tremor, irritability, proximal
  myopathy
• Ocular wide, staring gaze, lid lag

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Graves Disease

• Is the most common form of thyrotoxicosis (90)
• FM 4-51, 30-50 yoa
• Aetiology
• autoantibodies which stimulate the thyroid ( in
  contrast with Hashimotos)

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Graves Disease

• Thyrotoxicosis and goitre due to increased
  stimulation
• Infiltrative opthalmopathy exopthalmos
• Dermopathy ( pretibial myxedema) due to
  accumulation of GAGS

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Toxic Adenoma

• A benign tumour which secretes excess thyroid
  hormones
• Note there can also be malignant thyroid tumours
  but these are uncommon

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