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Exogenous triiodothyronine (T3) was substituted for levothyroxin or desiccated thyroid in 13 athyrotic patients previously treated for papillary, follicular, or mixed papillary-follicular carcinoma of the thyroid. After 4 weeks T3 therapy was discontinued and serial determinations of plasma thyroid stimulating hormone (TSH) concentrations were made. A roughly exponential rise in TSH values, corresponding to a doubling time of 2 days, was observed until a level of 40 microIU/ml was reached, after which the curve passed through a maximum at 20 days. The mean time required for a level of 50 microIU/ml was 11 days, and this time is suggested for TSH determination before I-131 imaging of patients with thyroid cancer. Plasma TSH values in eight patients following "total" thyroidectomy showed a much slower and more variable rise, with a mean doubling time of 7.6 days. Weekly TSH levels beginning at 15 days will provide a rational basis for I-131 imaging in this group, in whom a longer period of hypothyroidism will be required before imaging and therapy. Adherence to these protocols should minimize the duration of hypothyroidism in patients undergoing I-131 treatment of thyroid carcinoma.

Twenty patients with occult gastrointestinal bleeding underwent diagnostic evaluation with autologous TcRBC imaging, which identified a locus of bleeding in 19. Bleeding was demonstrated in the esophagogastroduodenal area in four patients, the cecum in four patients, the ascending colon or hepatic flexure in nine patients and the descending colon in two patients. In the six patients who underwent surgical intervention, the source of bleeding had been correctly located by imaging. The advantages of imaging over those of conventional arteriography included its minimally invasive nature and its ability to detect areas of intermittent minimal bleeding for a period of 30 hours. The major disadvantages of the technique were that localization of the site of bleeding was not as precise as that obtained with arteriography, imaging could not be used as a treatment modality and the nature of the lesion could not be defined. However, we believe that, as an adjunct to conventional angiography and other techniques, autologous TcRBC imaging will improve the localization of obscure foci of gastrointestinal bleeding. Studies are underway to determine the proper place of TcRBC imaging in the gastroenterologic armamentarium. Its noninvasive nature and sensitivity to intermittent hemorrhage suggest that it may become the initial investigative modality of choice in many instances.