The Thyroid Notes

The Thyroid

Tyrosine

Iodination

Monoiodotyrosine

Iodination

Diiodotyrosine

Triiodotyrosine T3 – active) Thyroxine (T4)

Monodeiodination

T3 acts via nuclear receptors to

• Increase cell metabolism

  • Stimulate glucose uptake and metabolism

  • Stimulate protein breakdown

Effects are usually catabolic e.g increased BMR, heat production + O2 consumption

• Increase catecholamine effects

  • Vital for normal growth and development

  • Absence causes physical and mental retardation (cretinism)

Anatomy

• The thyroid consists of two lobes connected by an isthmus

• Moves on swallowing

• Palpable in normal women

• Embriologically originates from base of tounge and descends to middle of neck

• Blood supply

  • Superior and inferior thyroid arteries

Hypothyroidism

- One of the most common endocrine conditions with UK prevalence of 1%

Pathophysiology

• Usually primary from underactive thyroid

• May be secondary to hypothalamic-pituitary disease (reduced TSH drive)

Causes

• Autoimmune

  • Hashimotos thyroiditis

    • Lymphocytic and plasma cell infiltration

    • Goitre

    • V. High antibody titres

  • Primary atrophic hypothyroidism

    • Common

    • Diffuse lymphocytic infiltration of thyroid = atrophy (no goiter)

• Acquired

  • Iodine deficiency

  • Destructive therapy (radioiodine, surgery) for cancer/hyperthyroidism

  • Drug induced

Clinical features

Investigations

• TFTs

  • High level serum TSH indicates primary hypothyroidism

  • Low free T4 confirms hypothyroid state

• FBC

  • Anaemia due to menorrhagia, or associated pernicious anaemia

• LFT

  • Increased aspartate transferase levels from muscle/liver

• Lipids

  • Hypercholesterolaemia

• U+E

  • Hyponatraemia due to increase in ADH and impaired free water clearance

Management

• Replacement therapy with levothyroxine (T4) for life

• Starting dose dependent on patient’s age and cardiac performance

• Usually 25-50ug/day, raising every month to achieve therapeutic level (usually 125ug/day)

• Patients with ischameic heart disease need regular ECGs as the dose is increased

• Assessment clinically 6 weeks post treatment starting and TFTs

• In pregnancy, doses need to be increased as high levels of TSH can cause cognitive impairment to the child

Myxoedema Coma

• Severe hypothyroidism may present with confusion or coma

• Rare

• Hypothermia is present and patient may have cardiac failure

Examination

• Goitre

• Cyanosis

• Heart failure

• Evidence of precipitants

Management

Bloods

• TFTs (T3, T4, TSH)

• ABG

Medication

• T3 orally or IV TDS

• Hydrocortisone 100mg IV TDS

Supportive measures

• O2 if cyanosed

• Cardiac monitoring

• Gradual rewarming

• Glucose infusion to prevent/correct hypoglycaemia

Hyperthyroidism (thyrotoxicosis)

Epidemiology

• 2-5% of all females

• Female>Male

• 99% due to intrinsic thyroid disease

• Pituitary disease is very rare

Aetiology and pathogenesis

Grave’s Disease

• Autoimmune

• Serum IgG antibodies bind to thyroid TSH receptor stimulating thyroid hormone production, behaving like TSH

• These TSH receptor antibodies can be detected in serum

• Associated with specific clinical features

  • Eye disease

  • Pretibital myxedema

Toxic multinodular goiter

• Seen in elderly + iodine deficient

• Nodules secrete thyroid hormone

• Relapses with medication alone – therefore radioactive iodine/surgery indicated

Toxic solitary adenoma/nodule (Plummer’s disease)

• Solitary nodule producing T3 and T4

• Rest of gland suppressed

Clinical features

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