This patient's clinical picture is consistent with euthyroid sick syndrome (also called nonthyroidal illness syndrome). Unless there is a strong suspicion of an underlying thyroid disorder that may be contributing to a patient's clinical findings, thyroid function tests should not be performed during critical illness. In hospitalized patients, especially ones as ill as the patient described here, thyroid function test results are highly likely to be abnormal. The first deviation that may occur during an acute illness is lowering of the serum total triiodothyronine (T₃) level. The serum thyroid-stimulating hormone (TSH) and serum free thyroxine (T₄) levels may also decline as the illness increases in severity. The pattern of these test results is often indistinguishable from that seen with central hypothyroidism. In fact, some controversy exists about whether the clinical picture of a low serum TSH level and low serum T₄ and T₃ levels is an adaptive response to the critical illness in order to alter the body's metabolism and thereby aid in recovery from the acute illness.

This patient does not have any definite findings of underlying Graves disease. The tachycardia and fever are likely the result of his severe infection. If he was also experiencing thyroid storm, high serum T₄ and T₃ levels would most likely be associated with the low serum TSH concentration.

The most common cause of hypothyroidism is Hashimoto thyroiditis. Physical examination findings can include a reduced basal temperature, diastolic hypertension, an enlarged thyroid gland, bradycardia, pallor, dry and cold skin, brittle hair, hoarseness, and a delayed recovery phase of deep tendon reflexes. Patients with Hashimoto thyroiditis have low T₄ and T₃ levels and elevated TSH. This patient's low TSH level is not compatible with Hashimoto thyroiditis.

This patient is unlikely to have subacute thyroiditis based on the timing of the illness. Although the infection could have triggered destruction of the thyroid, the subsequent release of preformed thyroid hormones into the serum should result in an elevation of serum T₄ and T₃ levels in the early phase of the disease, which is not consistent with the findings in this case.