Correct statements about thyroid function tests include which of the following?

A. Contraceptive pills and pregnancy increase the amount of thyroxin-binding globulin (TBG), and, consequently, the total T 4 level.

B. Anticonvulsive medications and chronic debilitating illnesses decrease the amount of TBG and, consequently, the total T 4 level.

C. Intravenous pyleography can lower the rate of active iodine uptake by the thyroid.

D. A triiodothyronine (T 3) suppression test that demonstrates nonsuppressibility of thyroid function is compatible with the diagnosis of Graves' disease, toxic adenoma, or functioning carcinoma.

DISCUSSION: Hormone binding proteins are the principal intravascular factors influencing total hormone concentration. Various factors may cause changes in the concentration of TBG. Contraceptive pills and pregnancy increase the amount of TBG, while anticonvulsive medications and chronic debilitating disease may decrease the amount of TBG. Administration of excess amounts of iodine may lower the rate of active iodine uptake in the thyroid gland. The thyroid suppression test is based on the principle that administration of thyroid hormone does not suppress the patient's thyroid function when normal homeostatic mechanisms are disrupted. Thyroid function is nonsuppressible in hyperthyroidism or in the presence of thyroid hormone–secreting tumors. Serum cholesterol, deep tendon reflex time, and cardiovascular functions can be measured to reflect hypothyroidism and hyperthyroidism but do not give an indication of the etiology of hypothyroidism.