This article discusses the reliability of laboratory-based diagnosis of hypothyroidism by means of combined determination of TSH and FT4. Differential diagnoses and supplementary tests are listed, and administrative and analytical errors reviewed. It is concluded that unexpected results should not be accepted without undertaking control analyses and supplementary examinations. Partial selective peripheral resistance to thyroid hormones (persons who have normal TSH and FT4 but need thyroxine therapy) is a very uncommon condition. Patients suspected of this disorder should be referred to their respective regional hospitals, and the results of the study should be published in international journals.