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

For a patient suspected of having hypothyroidism, the initial investigations depend on whether secondary thyroid dysfunction (pituitary disease) is suspected ¹.

• For adults when secondary thyroid dysfunction is not suspected: Consider measuring thyroid-stimulating hormone (TSH) alone ¹. If the TSH level is above the reference range, free thyroxine (FT4) should be measured in the same sample ¹.
• For adults when secondary thyroid dysfunction is suspected, or for children and young people: Consider measuring both TSH and FT4 initially ¹. If the TSH is below the reference range, free tri-iodothyronine (FT3) should also be measured in the same sample ¹.

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 ¹. If symptoms worsen or new symptoms develop, consider repeating the tests, but no sooner than 6 weeks from the most recent test ¹.