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Best Tests December 2005

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Thyroid Function Tests

When requesting thyroid function tests, bpacnz recommends

  • Asymptomatic patients are not screened for thyroid dysfunction
  • TSH is used as the sole initial test of thyroid function in most situations

Key points when requesting thyroid function tests

Testing asymptomatic patients

  • Routine or opportunistic screening of asymptomatic patients is not recommended
  • The return of positive results is low and there is controversy around the value of treatment in apparently healthy people

Monitoring patients on thyroxine

Non-pregnant patients:
  • Wait at least 6 weeks to test TSH after adjusting thyroxine dose
  • Monitor stable patients annually with TSH only
Pregnant patients:
  • Check TSH of hypothyroid women who are planning pregnancy
  • Check TSH and FT4 early in pregnancy, and at the start of trimesters two and three
  • Check thyroid function more frequently if there is a change in thyroxine dose

Monitoring anti-thyroid medication

  • Test TSH and FT4 until TSH normalises, then
  • Monitor every 2 months using TSH only

Untreated subclinical hypo- and hyperthyroidism

  • An abnormal TSH should be confirmed several months later
  • If still abnormal, monitor the TSH every 12 months unless symptoms develop
  • Patients with positive thyroid antibodies may need closer monitoring

Unwell patients

  • During illness, there may be transient changes in TSH, FT4 and FT3
  • Try to defer thyroid function testing until the illness has resolved

Patients on other drugs

  • Amiodarone: Patients on long term therapy should have 6-monthly TSH and FT4 tests
  • Lithium: Use TSH annually to check thyroid function

When to request both TSH and FT4

  • During pregnancy
  • Suspected non-adherence to thyroid replacement regimen
  • When a patient is suspected of having pituitary failure both TSH and FT4 should be requested, as often the patient has a normal TSH with a decreased FT4