Best Tests December 2005
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
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