THE USE OF RADIOACTIVE IODINE IN THE ASSESSMENT OF THYROID FUNCTION1

1. Radioactive iodine has been used to investigate the thyroidfunction of a group of 16 normal subjects, 30 patients in an"intermediate" group with non-toxic goitres, treated thyrotoxicosis,or anxiety states, 44 patients with thyrotoxicosis or toxicadenomata, four patients with myxoedema, one with acromegaly,and one with Hashimoto's thyroiditis. 2. The metabolism of orally administered radioactive iodinehas been studied in each group with respect to the rate anddegree of uptake of the isotope by the thyroid gland, the urinaryexcretion in the first 24 hours, the thyroid clearance rateof iodide from the plasma, and the amount of activity due tothe isotope found in the plasma 48 hours after the dose wasgiven. The proportion of the plasma activity which was due toradioactive iodine in protein-bound form has been estimated. 3. Characteristic patterns of behaviour of the administerediodine were found for each group in respect of each criterioninvestigated. The gland uptake at its peak value showed a rangeof 21 to 54 per cent, with a mean of 38 per cent, in the normalgroup, 24 to 69 per cent, with a mean of 48 per cent, in theintermediate group, and 56 to 92 per cent, with a mean of 74per cent, in the toxic group. The uptake in the myxoedematouspatients ranged from 4 to 23 per cent. Gland uptakes at 24 hoursshowed similar ranges, but there was more overlap between thetoxic group and the intermediate and normal groups. 4. The time was measured to the moment at which the thyroidgland had absorbed half the iodine retained at the peak pointof the curve. A time to half-peak in excess of 160 minutes wascharacteristic of normal thyroid function, and values below100 minutes were characteristic of thyrotoxicosis. Intermediatetimes occurred both in clinically normal and in thyrotoxic subjects. 5. Thyroid iodide clearance rates in normal persons were commonlybelow 50 ml. per minute, and usually well above 80 ml. per minutein the presence of clinical thyrotoxicosis, but intermediatevalues were found in non-toxic goitre, treated thyrotoxicosis,anxiety states, and the milder degrees of thyro-toxicosis. 6. The 24-hour urinary excretion of the isotope showed thatexcretions of under 20 per cent, of the dose were usual withclinical evidence of toxicity, and excretions of over 35 percent, with normal function. Intermediate excretions were foundboth in mild thyrotoxicosis and in non-toxic goitre. 7. Tests which measured the rate and degree of iodide concentrationin the thyroid gland showed a rough correlation with the clinicalseverity of the disease. 8. Estimation of the protein-bound plasma activity 48 hoursafter administration of the dose showed a very sharp divisionbetween normal and thyrotoxic subjects. Values in excess of.0.4 per cent, of the dose per litre of plasma were found inthyrotosdcosis, and values below 0.1 per cent, were found innon-toxic goitres, treated thyrotoxicosis, anxiety states, andnormal subjects. Estimations of the total plasma activity at48 hours provided valuable but less definite diagnostic data,values in excess of 0.7 per cent, per litre being usually foundin thyrotoxicosis, and seldom found in euthyroid persons. 9. The estimation of the protein-bound plasma activity 48 hoursafter a tracer dose of radioactive iodine is essentially a measureof the amount of circulating thyroxine. It gives valuable diagnosticinformation, and is suitable as a screening procedure for out-patients.