What are the recommended initial investigations for a patient suspected of having hypothyroidism?

Guideline-aligned answer with reasoning, red flags and references. Clinically reviewed by Dr Kola Tytler MBBS CertHE MBA MRCGP.

Posted: 14 August 2025 Guideline-Aligned (High Confidence) Clinically Reviewed
Dr Kola Tytler MBBS CertHE MBA MRCGPClinical Lead • iatroX

For a patient suspected of having hypothyroidism, the initial investigations depend on whether secondary thyroid dysfunction (pituitary disease) is suspected .

  • For adults when secondary thyroid dysfunction is not suspected: Consider measuring thyroid-stimulating hormone (TSH) alone . If the TSH level is above the reference range, free thyroxine (FT4) should be measured in the same sample .
  • For adults when secondary thyroid dysfunction is suspected, or for children and young people: Consider measuring both TSH and FT4 initially . If the TSH is below the reference range, free tri-iodothyronine (FT3) should also be measured in the same sample .

It is important to ask adults, children, and young people about their biotin intake, as high consumption from dietary supplements can lead to falsely high or low test results . If symptoms worsen or new symptoms develop, consider repeating the tests, but no sooner than 6 weeks from the most recent test .

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