⚠️ Do not request thyroid function tests routinely in ITU/HDU or other acutely ill inpatients unless there is a specific clinical suspicion of thyroid disease. Abnormal results are often due to Non-Thyroidal Illness Syndrome (NTIS) and may normalise after recovery. If thyroid disease is still suspected, repeat TFTs after recovery rather than diagnosing chronic thyroid disease during the acute illness.

📖 About

Non-Thyroidal Illness Syndrome (NTIS), also called euthyroid sick syndrome, describes abnormal thyroid function tests in people with significant acute or chronic illness without primary thyroid disease.
The commonest biochemical pattern is low T3, with normal or low T4 and a TSH that may be low, normal, or mildly raised.
It is common in severe illness, including sepsis, trauma, starvation, and multiorgan failure.

🧬 Pathophysiology

Reduced peripheral conversion of T4 to T3 due to altered deiodinase activity during illness.
Increased production of reverse T3 may occur because thyroxine is preferentially shunted away from active T3 production.
Hypothalamic-pituitary changes alter TSH secretion and pulsatility, so TSH may be transiently suppressed early in illness.
Inflammation, glucocorticoids, dopamine, fasting, and critical illness all contribute to disturbed thyroid hormone metabolism.

🔑 Causes / Associations

Severe systemic illness: sepsis, trauma, burns, myocardial infarction, renal failure, liver disease.
Starvation or major calorie restriction.
Metabolic and endocrine stress states such as DKA.
Drug effects, especially glucocorticoids and dopamine, which can suppress TSH.

🩺 Clinical Features

The patient is usually not clinically hypothyroid or hyperthyroid; symptoms are mainly due to the underlying illness.
NTIS is usually suspected when abnormal TFTs are found in a patient who is acutely unwell and has no convincing thyroid history.
During recovery, TSH may transiently rise, which can mimic subclinical hypothyroidism.

🔎 Investigations

T3: often reduced first and most consistently.
Free T4: may be normal or low, especially in more severe illness.
TSH: may be low, normal, or mildly raised; an abnormal TSH in hospital does not reliably diagnose thyroid disease by itself.
Reverse T3: may be raised, but this is not a routine UK investigation and is generally not needed in standard clinical practice.
Cortisol: consider only if adrenal insufficiency is suspected clinically.
Repeat TFTs after recovery if thyroid disease remains a concern.

⚕️ Management

👀 Do not treat NTIS itself with thyroid hormone routinely.
🩺 Treat the underlying illness and correct major physiological derangements.
🚫 Avoid diagnosing chronic thyroid disease solely from TFT abnormalities during severe acute illness unless the clinical picture strongly supports it.
🔁 Repeat thyroid function tests after recovery if results remain clinically relevant.

💡 Teaching Pearls:
– In hospitalised patients, TSH is often the least reliable test when interpreted out of clinical context.
– Low T3 is the classic biochemical change in NTIS.
– A transient TSH rise during recovery is recognised and should not be overcalled as new hypothyroidism.
– The key bedside skill is to decide whether there is real clinical evidence of thyroid disease, not just an abnormal number.

📚 References

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Sick Euthyroid Syndrome ✅