Late Endocrine Reactions

Thyroid Late Effects

The thyroid is an endocrine organ that is located in the neck. Although relatively tolerant in adults (TD5/5 of 45 Gy), it is susceptible to dysfunction later in life in young adults and children who receive radiation to the neck. The development of toxicity often takes many years (more than 5).

Clinical Syndromes

There are three types of thyroid disease that can be caused by radiation:

  • Hyperthyroidism (symptoms of anxiety, insomnia, sweating, temperature intolerance)
  • Hypothyroidism (symptoms of depression, fatigue, malaise, dementia)
  • Thyroid cancer

Pathophysiology

Following radiation exposure of the thyroid gland in rats, the following observations have been noted histopathologically:

  • An increase in the hyalinisation of blood vessel walls
  • Necrosis of follicles
  • A strong inflammatory reaction

Hypothyroidism is likely to result from a combination of follicle loss and from vascular insufficiency to the thyroid gland.
Hyperthyroidism occurs due to thyroiditis or Grave's disease following radiation exposure. Thyroiditis is most likely due to the inflammatory response seen histologically following radiation, perhaps in a similar way to radiation pneumonitis in other tissues. Grave's Disease, an autoimmune condition where auto-antibodies are formed against the TSH receptor, may be caused due to increased exposure of the immune system of thyroid auto-antigens.
Thyroid cancer is greatly increased in likelihood if the thyroid gland is radiated at a young age; in adults the thyroid does not seem more prone to cancer than other tissues. In general, the cancers that arise from radiation induction are well differentiated and easily treated; about 5% of patients with a radiation induced malignancy of the thyroid die from their condition. Increased rates of malignancy in children have been seen from nuclear weapon attacks in Japan, nuclear testing in the Marshall Islands, and in children exposed to radiation for enlarged thymus, tonsillitis (wtf?) and malignancies.

Tolerance Doses

Hall lists the TD5/5 as 45 Gy, and the TD50/5 as 140 Gy; this may simply mean that thyroid problems develop more slowly and do not become apparent after 5 years in some cases.


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