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Thyroid Function
 
 

Hypothyroidism

 
 
 
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Overview

Hypothyroidism refers to underactivity of the thyroid, with reduced circulating thyroid hormones. This is usually accompanied by typical signs and symptoms of hypothyroidism.
  • TFT Interpretation

  • Primary hypothyroidism - elevated TSH with reduced T4
  • Subclinical hypothyroidism - elevated TSH with normal T4
  • Central hypothyroidism - reduced TSH with reduced T4

Manifestations

  • Clinical Features

  • Symptoms

  • Weakness / lethargy / mental sluggishness
  • Weight gain
  • Cold intolerance
  • Dry, coarse skin
  • Constipation
  • Menorrhagia
  • Alopecia
  • Signs

  • Bradycardia
  • Psychomotor retardation
  • Impaired cognition
  • Cool, dry skin
  • Facial oedema
  • Loss of the outer ⅓ of the eyebrow
  • Tongue enlargement
  • Alopecia
  • Hyporeflexia
  • Non-pitting oedema

Primary Hypothyroidism

Hypothyroidism that originates from thyroid tissue.
  • Look For

  • Elevated TSH with reduced T4.
  • Causes of Primary Hypothyroidism

  • Autoimmune thyroiditis - Hashimoto’s, atrophic
  • Hypothyroid phase of thyroiditis
  • Iatrogenic - radioiodine therapy, thyroidectomy, external irradiation of the neck
  • Drugs - amiodarone, lithium, interferons, interleukin-2, iodide
  • Iodine deficiency
  • Thyroid infiltration - amyloidosis, tumour
  • Congenital hypothyroidism

Subclinical Hypothyroidism

Patients with subclinical hypothyroidism have TFT abnormalities without overt clinical features of hypothyroidism, or with nonspecific symptoms.
  • Look For

  • Elevated TSH with normal T4
  • No clinical features of hypothyroidism
  • Causes of Subclinical Hypothyroidism

  • As per primary hypothyroidism.
  • Differential Diagnosis of TFT Findings

  • Poor compliance with thyroxine
  • Assay interference
  • Drugs - amiodarone, sertaline
  • Nonthyroidal illness (recovery phase)
  • TSH receptor defects
  • TSH resistance
  • Pearls

  • Take a careful medication history
  • Ask about recent illnesses

Central Hypothyroidism

Pituitary or hypothalamic lesions are a less common cause of hypothyroidism.
  • Look For

  • Reduced / normal TSH with reduced T4.
  • Causes of Central Hypothyroidism

    Pituitary or hypothalamic pathology.
  • Tumour
  • Trauma
  • Surgery
  • Radiation
  • Infection
  • Infarction
  • Infiltration
  • Differential Diagnosis of TFT Findings

  • Nonthyroidal illness
  • Recent treatment of hyperthyroidism
  • Congenital TSH or TRH deficiency
  • Assay interference
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