Euthyroid Sick Syndrome
Euthyroid sick syndrome — also called non-thyroidal illness syndrome — describes abnormal thyroid test results that occur during a significant illness, surgery, starvation or major stress, even though the thyroid gland itself is functioning normally.
It is important to recognise because it can be mistaken for thyroid disease and lead to unnecessary treatment.
Last medically reviewed: 15 November 2025
What the tests show
Typical patterns include a low T3, low or low-normal T4 and free T4, and an inappropriately normal or slightly low TSH. The changes reflect the body’s adaptation to illness rather than thyroid dysfunction.
The severity of the abnormalities often tracks with how unwell the person is.
Why it happens
Illness alters the proteins that carry thyroid hormone, changes how tissues convert T4 to T3, and shifts the feedback on the pituitary. These are physiological responses, not a diseased thyroid.
Medications used in illness can contribute to the pattern as well.
What to do
The main action is to avoid misdiagnosis: thyroid tests taken during acute illness should generally be repeated after recovery before any diagnosis is made or treatment started.
Treatment is aimed at the underlying illness. Thyroid hormone is not routinely given for this pattern and may be harmful.
Frequently asked questions
Do I have a thyroid problem?
Abnormal thyroid tests during serious illness often do not mean thyroid disease. They should be repeated after recovery to know for certain.
Should these results be treated?
Not routinely — treating the underlying illness is the priority. Thyroid hormone is not usually given and may cause harm.
When should I retest?
Usually several weeks after you have recovered from the acute illness or stress, as advised by your clinician.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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