Sixty-three elderly patients (aged more than 65 years) who manifested primary hypothyroidism during 4 1/2 years from September 1980 to March 1985 were studied. Patients were divided into two groups depending on the presence or absence of chronic associated disorder at the time of diagnosis. Patients in the sick group were required to consume several medications throughout the study period in addition to L-thyroxine for their total therapeutic management. Subjects in the healthy group required L-thyroxine administration alone for their therapy. Prior to institution of L-thyroxine therapy, serum thyroxine (T4) was not significantly different in the two groups. However, serum triiodothyronine (T3) and thyroid-stimulating hormone (TSH) levels were significantly lower and T3 resin uptake was significantly higher in the sick group compared with the healthy patients (P less than .01 for all comparisons). Furthermore, these differences in T3, TSH concentration, and T3 resin uptake values appeared to persist on achieving euthyroid state. The optimal daily L-thyroxine dose was markedly lower (97 +/- 3 micrograms) in the sick patients compared with the healthy group (144 +/- 3 micrograms) as well as with the younger counterparts reported in the literature (150 +/- 8 micrograms). These findings indicate that the decrease in optimal daily L-thyroxine dosage reported in previous studies is not a universal finding in all elderly hypothyroid patients; the decrease is present only in patients with associated chronic disorders, and hence may be attributed to the presence of an associated chronic disorder or medications consumed for treatment of these disorders rather than old age.