[Diagnostic value of blood TSH levels in seriously ill patients]

Undetectable serum TSH values measured by immunoradiometric methods are highly suggestive of hyperthyroidism. However, serum TSH levels can also be very low in severely ill patients and in this case thyroid disease can be ruled out by thyroid hormone measurements. The authors have determined the frequency of low thyroid hormone values and of low serum TSH measurements in hospitalized patients. They investigated 276 euthyroid patients, 21 of whom were found to have decreased values for serum total T4 and free T4 index (FTI). Serum TSH was below the lower limit of detection (less than 0.2 mU/l) in 4 of these 21 patients (19%) and normal in 17 others. Serum TSH measurements are therefore considered more reliable than thyroid hormone measurements in ruling out primary hypothyroidism. Interestingly, serum TSH was also found to be unmeasurable in 7 of 248 patients with normal total serum T4 and FTI (3%). An unmeasurable TSH value is therefore not synonymous with low T4 in sick patients, which indicates that both measurements are needed in this category of patients. Data are also included on 7 patients with high serum total T4 and FTI but still normal serum T3, in whom as measurable serum TSH concentration rules out hyperthyroidism.