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hypothyroidism

hypothyroidism

Introduction

  • hypothyroidism is a common chronic condition with an incidence of 3.5/1000 women and 0.6/1000 men.
  • prevalence increases with age
  • finding high TSH levels in a symptomatic patient confirms the diagnosis and a cause is readily found.
  • lifelong thyroxine Rx relieves symptoms and restores “normal” thyroid hormone funtion.
  • commencing thyroxine can aggravate cardiac disease

Aetiology

  • endemic iodine deficiency
    • interestingly, white people caused endemic cretinism affecting 15% of children in Papua New Guinea from the 1950's onwards when they supplied salt to the locals which saved them a 2 day trek to get their usual iodine-rich salt from salt pools. See Life in the fast lane blog on this story
    • endemic cretinism results from maternal hypothyroidism as the early fetus cannot make its own T4, and affects development of the fetus throughout its development, especially brain development.
  • autoimmune thyroid disease
  • thyroidectomy
  • radiotherapy (radio-iodine or external beam radiotherapy)
  • drug-induced
      • lithium is taken up and concentrated by the thyroid, and like iodine, inhibits release of T4 resulting in a compensatory increase in TSH but usually euthyroid clinical status, however, many develop a goitre over time and those predisposed may develop clinical hypothyroidism.
      • acute lithium toxicity with its slow elimination of lithium, may precipitate hypothyroidism
    • iodine-containing preparations
      • amiodarone inhibits the peripheral conversion of T4 to T3, effectively reducing the amount of active hormone
      • it also inhibits uptake of thyroid hormone by cells, so measured levels may increase but tissue effects decrease
      • it is directly cytotoxic to follicular cells inducing thyroiditis, and it transiently increases TSH
      • net effect is that 20% on long term Rx develop hypothyroidism while 3% develop hyperthyroidism, though most cases are mild.
  • congenital hypothyroidism
    • congenital hypothyroidism differs from endemic cretinism in that as it is caused by failure of the fetus to produce its own thyroid hormones in mid-late gestation, brain development in early gestation is not affected as this is dependent on maternal thyroid hormones.
  • disorders of thyroid hormone metabolism
  • secondary hypothyroidism due to some pituitary and hypothalamic diseases

main clinical features of hypothyroidism

  • myxoedema refers to the thickened, non-pitting, oedematous skin and subcutaneous tissues seen in most patients with advanced hypothyroidism.
  • commonly, myxedema is used to refer to the clinical picture of hypothyroidism:
    • lethargy, weakness, thinning hair, cold intolerance, weight gain, constipation, depression, hypothermia, delayed deep tendon reflexes, waxy, non-pitting oedema, pleural or pericardial effusions, husky voice, periorbital oedema, and bradycardia.
  • myxoedema coma is the most extreme form and is often precipitated by an acute stress such as anaesthesia, cardaiac failure, stroke, or unusual cold exposure, and has a 50-70% mortality.
  • one should have a high suspicion of hypothyroidism in a patient aged > 55 years with altered mental state who is cold and has non-pitting oedema.

diagnosis of hypothyroidism

  • persistently high TSH with low free T4 suggest primary hypothyroidism
    • free T3 levels don't add much
  • persistently high TSH (usually to 5-10IU/L) with normal free T4 suggest subclinical primary hypothyroidism and typical symptoms may be absent.
  • low free T4 without elevated TSH levels suggest secondary hypothyroidism due to pituitary disorders
  • low free T3 without elevated TSH levels has been called Low T3 syndrome
    • usually also have higher levels of a hormone known as reverse T3
    • appears to be present in less than 20% of CFS and it might play a role in worsening conditions like kidney disease or serious infections
  • if cause is not obvious then:
    • high titres of antithyroid antibodies (thyroid peroxidase, antimicrosomal, or antithyroglobulin) indicate an autoimmune cause

Thyroxine Rx

  • half life 7-10 days but a much longer biological effect
  • once daily dosing
  • dose is dependent on body weight and age
  • children require larger doses per kg than adults who usually require 1.6 microgram/kg/day
  • most adults will maintain euthyroidism on 100-200 microgram/day
  • there may be a decline in thyroxine requirements in the elderly
  • a BMJ study in 2011 showed fracture risk of 2.5 to 3.5x in those over 70yrs taking > 0.044mg/day thyroxine - see also osteoporosis
  • a month's supply can be kept at room temperature - advice to refrigerate risks deterioration from moisture.

starting Rx

  • rate of introduction of thyroxine should be determined by duration of hypothyroidism & presence or risk of ischaemic heart disease (IHD) or congestive cardiac failure.
  • healthy adult patients who have undergone thyroidectomy can immediately start at or just below their predicted daily dose of 100-200 microgram.
  • elderly pts and those with known heart disease should start with 25 microgram per day for 3-4 weeks with reassessment and further increments of 25 microgram per day every 3-4weeks as indicated until predicted dose is reached.
    • worsening symptoms of cardiac disease should be controlled before increasing dose, and dose reduction may be needed.
  • other patients can be started on 50 micrograms/day and increase by 50 micrograms/day every 3-4 weeks.
  • pts should feel symptomatic improvement within 2wks starting Rx but full benefit may take 3-4 months
  • target TSH should be at lower end of normal range (0.4-5IU/L)
    • takes at least 4 weeks for TSH to stabilise after a change in dose
  • consider variable daily dosing to minimise cutting the tablets

References

hypothyroidism.txt · Last modified: 2018/03/23 07:16 by gary1