A patient who is in ICU is found to have a slightly low TSH, low T3 and normal T4. She has no history of thyroid disease. What is the most likely cause? Question 22Answer a. Hyperparathyroidism b. Subclinical hyperthyroidism c. Pituitary Hypothyroidism d. Euthyroid sick syndrome e. Hypothyroidism
The correct answer and explanation is:
Correct Answer: d. Euthyroid sick syndrome
Explanation:
Euthyroid sick syndrome, also known as non-thyroidal illness syndrome (NTIS), is the most likely diagnosis in a critically ill patient with abnormal thyroid function tests but no underlying thyroid disease. This condition is commonly seen in hospitalized patients, especially those in the intensive care unit (ICU) due to severe illness, trauma, or stress.
In euthyroid sick syndrome, thyroid hormone levels are altered as part of the body’s response to systemic illness. Typically, the first change is a reduction in the level of triiodothyronine (T3), a phenomenon known as “low T3 syndrome.” This occurs due to decreased peripheral conversion of thyroxine (T4) to T3. T4 levels are usually normal or low-normal in the early phase, and may decrease further in prolonged illness. Thyroid stimulating hormone (TSH) is often normal or slightly low initially, and may become slightly elevated during the recovery phase.
The laboratory findings in this patient – low T3, normal T4, and slightly low TSH – are characteristic of euthyroid sick syndrome. This condition is not caused by intrinsic thyroid or pituitary disease but rather is a consequence of altered hormone metabolism due to the critical illness itself. Importantly, these changes do not require thyroid hormone replacement therapy unless true primary thyroid dysfunction is also present.
Subclinical hyperthyroidism and hypothyroidism are less likely because they typically do not present with this pattern of hormone changes. Pituitary hypothyroidism would cause low TSH with low T4 and T3, not just isolated low T3. Hyperparathyroidism is unrelated to thyroid hormones and would not explain the thyroid function test abnormalities.
Therefore, the most appropriate diagnosis is euthyroid sick syndrome.