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The objective of this study was to prospectively determine the efficacy of the results of routine testing of thyroid functions in patients admitted to a general medical ward. Blood for thyroid function tests was drawn on admission as part of the laboratory screening panel from 270 consecutive patients. Fifty-one were excluded due to recognition of thyroid-related problems by the clinical staff. 138 patients (63%) had normal free thyroxin (FT4) levels, one patient had hyperthyroidism, 15 patients (6.8%) had laboratory primary hypothyroidism and 65 patients (29.6%) had equivocal results (FT40.9 ng/dL and TSH5.0 mmu/L). In eight out of 31 patients in the latter group, the thyroid releasing hormone (TRH) test revealed primary or secondary hypothyroidism. A total of 55 patients (25.1%) expired within one year. The mortality rate was significantly higher among the low FT4 patients (p0.01), and was independent of age and sex. We conclude that inclusion of FT4 in a laboratory screening panel will reveal a large patient population with abnormal results. However, establishing the diagnosis requires additional tests. Therefore, screening for occult hypothyroidism among a select population, not the acutely sick, populations may be more efficient.