An investigation of the value of the PBI and of the tri-iodothyronine (T,) test in an endemic goiter area shows that in euthyroid and hypothyroid cases PBI gives pathologically raised values in 30%. The Ts test in the same material gave incorrect values, the figure usually being too low in 8 % of the euthyroid cases. In the whole material (340 patients) PBI gave incorrect information in 28%, the Ts test in 9% and "free thyroxin index" (PBI x T3) in 7 % . In an endemic goiter area the diagnosis of the functional condition of the thyroid is considerably more difficult since tracer-iodine examination as well as PBI determination often give values expressing hyperthyroidism in euthyroid cases. Clinical assessment of the patients is therefore of specially great value in an endemic goiter area.