Hyperthyroidism can cause asthma to worsen, such that analysis of thyroid function has been recommended when the course of asthma is unfavorable. The aim of this study was to determine whether systematic analysis of thyroid function is useful for all patients with difficult-to-manage asthma. For prospective study, we enrolled 48 asthmatics whose condition had deteriorated due to no known cause. All patients were studied as follows: a) all were assessed for thyrotropin (TSH) levels and, if alterations were detected, we ordered analysis of free thyroxin (FT4), and b) case histories were taken to rule out the existence of active thyroid disease. Nine patients (19%) were suspected of thyroid disease. TSH levels were abnormal in only 5 (10%) patients in this group (low in four and high in one). Hyperthyroidism was confirmed in only 3 patients (6%) after high FT4 levels were detected. Although the frequency of hyperthyroidism in the sample studied is higher than that described for the general population, the systematic investigation of thyroid dysfunction in all patients with difficult-to-manage asthma does not appear justified given that disease could be demonstrated in only some of those who were clinically suspected of thyroid disease.