The use of different radioiodines and technetium for the evaluation of the thyroid functions has been discussed. Preparation oe labelled iodotyrosines and labelled throxine is mentioned for the convey of different radiochemical tests and for the use of reference standards, necessary for the identification of separated amino acids. Paper chromatography and paper electrophoresis techniques are used for the identification and determination of iodinated amino acids or homones, either in the free state or bound to the proteins. Tc99mis employed to substitute radioiodine in the study of uptake measurements of the thyroid gland and to evaluate the excretion of the isotope in urine. Thiourea-S35is employed, for the first time to human, to study the metabolism of the substance in different thyroid disorders.