Hyperthyroidism

Hyperthyroidism refers to increased production and secretion of thyroid hormone from the thyroid gland.

Terminology

Hyperthyroidism is not synonymous with thyrotoxicosis, the latter referring to a clinical syndrome of excess thyroid hormone ¹. 

Clinical presentation

Hyperthyroidism may be accompanied by the signs and symptoms of thyrotoxicosis (e.g. heat intolerance, weight loss, tremor). However, it is also possible to have a subclinical form of hyperthyroidism (i.e. lacking thyrotoxicosis), but this is less common ¹.

ECG

• sinus tachycardia
• high voltage QRS complexes
• atrial fibrillation

Thyrotoxicosis predisposes patients to a variety of tachydysrhythmias, as well as atrial and ventricular ectopy ²:

• paroxysmal supraventricular tachycardia (PSVT)
• non-specific ST segment and T wave changes
  • elderly patients may have rate-related ischemia
• premature ventricular complexes (PVCs)

Pathology

Etiology

There are numerous causes of hyperthyroidism ¹:

• increased thyroid gland stimulation
  • TSH receptor stimulating antibodies: Graves disease (most common cause of hyperthyroidism in developed countries ⁵)
  • hCG secretion (e.g. trophoblastic tumors)
  • excess TSH secretion (e.g. TSH-secreting pituitary tumors)
• independent thyroid function
  • TSH receptor gene activating mutations (e.g. functioning adenoma, multinodular goiter)
• thyroid tissue inflammation and subsequent release of stored thyroid hormone
  • autoimmune (e.g. acute Hashimoto thyroiditis, postpartum thyroiditis)
  • viral (e.g De Quervain thyroiditis)
  • infectious (e.g. bacterial)
  • iatrogenic
    • radiation thyroiditis
    • thyroid surgery
    • drug-induced (e.g. amiodarone)
• extrathyroid source
  • dietary (e.g. factitious hyperthyroidism, hamburger thyrotoxicosis ⁴)
  • neoplastic (e.g. functional thyroid cancer metastases, pituitary adenoma, struma ovarii)
• iodine exposure
  • iatrogenic (e.g. iodine contrast, iodine-containing drugs)
• pseudohyperthyroidism
  • high intake of the vitamin biotin may interfere with thyroid hormone assays ³