Serial TSH determination after T3 withdrawal or thyroidectomy in the therapy of thyroid carcinoma.

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.