Non-Thyroidal Illness Syndrome
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Direct answer
Non-thyroidal illness syndrome is a pattern of altered thyroid tests in severe systemic illness, with low serum T3, raised reverse T3 and usually normal or low T4 and TSH, despite a normal thyroid gland. It reflects reduced peripheral conversion of T4 to T3 and is an adaptive response, so treatment targets the underlying illness rather than the thyroid.
What you must remember
- Low T3: the most consistent abnormality, from reduced type 1 deiodinase activity converting T4 to T3 in peripheral tissues.
- Reverse T3: raised through type 3 deiodinase activity, which converts T4 into the inactive isomer reverse T3.
- TSH: normal or low in severe illness, with a transient rise possible during recovery that can mimic hypothyroidism.
- Free T4: normal in most patients but low in severe critical illness, so free T4 measurement is essential.
- Not a thyroid disease: the gland is normal, and the pattern appears in starvation, fasting, critical illness and chronic kidney or liver disease.
- Drug effects: glucocorticoids and dopamine suppress TSH, so a low TSH on routine testing may be drug-induced and does not by itself mean thyrotoxicosis.
- Management: treat the underlying illness, because routine thyroid hormone replacement has not improved outcomes in critical illness.
Telling the pattern apart from primary thyroid disease
The same low T3 can come from illness, starvation or primary hypothyroidism, so the pattern is judged by free T4, TSH and the clinical context together. Each test answers a different question, and reading them as a set prevents an unnecessary diagnosis.
| Feature | Non-thyroidal illness | Primary hypothyroidism | Hyperthyroidism |
|---|---|---|---|
| TSH | Normal or low | Raised | Suppressed |
| Free T4 | Normal, or low in severe illness | Low | Raised |
| T3 | Low | Low or normal | Raised |
| Clinical picture | Acute or chronic systemic illness | Fatigue and cold intolerance | Weight loss, tremor and tachycardia |
A persistently abnormal thyroid profile in a well person points to primary thyroid disease rather than illness. In an acutely unwell patient, a normal free T4 with low T3 and a suppressed or normal TSH should prompt repeat testing after recovery. Low T3 alone is never an indication to start treatment, and serial tests over days show whether the pattern is evolving with recovery or persists as true primary disease.
Where the exam tries to catch you
Most errors come from treating a laboratory pattern as a thyroid disease.
- Trap: Starting levothyroxine for low T3 in an ICU patient. Correct: Low T3 is an adaptive response, and replacement has not improved outcomes in critical illness.
- Trap: Diagnosing hyperthyroidism from a suppressed TSH during acute illness. Correct: Severe illness and glucocorticoids suppress TSH, so free T4 and repeat testing decide the diagnosis.
- Trap: Calling a raised TSH during recovery primary hypothyroidism. Correct: A transient TSH rise during recovery is expected, and repeat testing after recovery separates it from true hypothyroidism.
Frequently asked questions
Why is T3 low in non-thyroidal illness syndrome?
T3 is low because peripheral conversion of T4 to T3 falls, with reduced type 1 deiodinase activity, while type 3 deiodinase activity raises reverse T3 production. The thyroid gland itself is normal in non-thyroidal illness syndrome.
Does non-thyroidal illness syndrome need thyroid hormone treatment?
Thyroid hormone treatment is not recommended in non-thyroidal illness syndrome, because the pattern is an adaptive response to critical illness. Replacement has not improved outcomes, so management focuses on treating the underlying illness and repeating thyroid tests after recovery.
Which thyroid test best distinguishes non-thyroidal illness from primary hypothyroidism?
Free T4 and TSH together are the most useful tests, since primary hypothyroidism shows a raised TSH with low free T4. Non-thyroidal illness usually shows normal or low TSH, and the tests are often repeated after the acute illness settles.
Why can TSH rise during recovery from severe illness?
TSH can rise transiently during recovery from non-thyroidal illness, sometimes above the reference range, which mimics hypothyroidism. This rise is expected, so thyroid tests should be repeated after recovery before a diagnosis of primary hypothyroidism is made.