In a critically ill patient with TSH 2.0 mIU/L, FT4 8 pmol/L, FT3 1.5 pmol/L, what is the most likely diagnosis?

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Multiple Choice

In a critically ill patient with TSH 2.0 mIU/L, FT4 8 pmol/L, FT3 1.5 pmol/L, what is the most likely diagnosis?

In critical illness, thyroid hormone levels can look abnormal even when the thyroid itself isn’t diseased—this is the euthyroid sick syndrome (non-thyroidal illness). The body changes thyroid hormone production and metabolism during severe illness, especially lowering the active hormone T3 because of reduced peripheral conversion from T4 and altered binding. Here, the key pattern is a markedly low FT3 (1.5 pmol/L) with a normal TSH (2.0 mIU/L) and a low-ish FT4 (8 pmol/L). The low T3 in the setting of a non-elevated TSH suggests the thyroid isn’t overactive or failing due to a primary thyroid disorder; rather, the illness is driving these changes. In Graves disease, you’d expect elevated FT4 and suppressed TSH; in Hashimoto’s, high TSH with low FT4; in true hyperthyroidism, elevated FT3/FT4 with suppressed TSH. So the presentation fits sick euthyroid syndrome.

In critical illness, thyroid hormone levels can look abnormal even when the thyroid itself isn’t diseased—this is the euthyroid sick syndrome (non-thyroidal illness). The body changes thyroid hormone production and metabolism during severe illness, especially lowering the active hormone T3 because of reduced peripheral conversion from T4 and altered binding.

Here, the key pattern is a markedly low FT3 (1.5 pmol/L) with a normal TSH (2.0 mIU/L) and a low-ish FT4 (8 pmol/L). The low T3 in the setting of a non-elevated TSH suggests the thyroid isn’t overactive or failing due to a primary thyroid disorder; rather, the illness is driving these changes. In Graves disease, you’d expect elevated FT4 and suppressed TSH; in Hashimoto’s, high TSH with low FT4; in true hyperthyroidism, elevated FT3/FT4 with suppressed TSH. So the presentation fits sick euthyroid syndrome.

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