Subclinical hypothyroidism was detected in 5.7% of 853 patients referred for ambulatory examination to a department in Prague. In patients referred with a mild degree of subclinical hypothyroidism with an evaluated level of thyrotropic hormone in serum (TSH) no significant deviations of serum thyroxine and triiodothyronine (T4 and T3) were found nor different values of the Achilles tendon reflex time, as compared with patients with normal thyroid function. In case of a more marked grade of subclinical hypothyroidism with an elevated TSH level without stimulation the patients have on average significantly lower T4 vales, significantly longer Achilles tendon reflex times and preejection stage of left ventricular contraction, and of Rodbard's Q-Kd interval (Q-Kd) than euthyroid patients. The values are, however, still within the reference range for normal function and in individual cases cannot be decisive for establishment of the diagnosis. The authors discuss the differential diagnosis and views on treatment of this disease.