Pseudo hypothyroidism may be defined as a deceased T4 level in a euthyroid person. This may occur with (1) decreased TBG or TBG binding (congenital or drug induced), (2) certain medications (e.g., phenytoin, lithium, dopamine, corticosteroid), (3) some patients with severe non thyroidal illness, (4) some clinically euthyroid patients with Hashimoto’s thyroiditis, (5) after recent therapy of hyperthyroidism or thyroid cancer with radioactive iodine (some patients eventually develop true hypothyroidism), (5) Cushing’s syndrome, (7) in some patients with SSKI therapy, and (8) severe iodine deficiency.

Most of these conditions have been discussed previously (see the box). The TSH levels are normal when the TBG level is decreased and in most of the drug-induced causes of nonhypothyroid T4 decrease. Cushing’s syndrome is associated with decreased T4, T3-RIA, TSH, and TBG levels in most patients. The TRH test results usually show a blunted TSH response. The TSH level becomes elevated in some (usually a minority) of patients with the remainder of the conditions listed above (also see the box); TSH values are usually (but not always) less than 3 times the upper limit of the reference range and most frequently are less than twice that limit. Decreased T4 levels with elevated TSH levels in thyroiditis and following radioiodine therapy might be considered true hypothyroidism, even if it is only temporary, especially since some of these patients go on to develop clinical as well as laboratory hypothyroidism. Lithium carbonate therapy might be included in pseudo hypothyroidism since the abnormalities it produces are reversible. On the other hand, in some cases of long-term therapy the laboratory abnormalities persist after medication is stopped. About 8%-10% of patients have decreased T4 levels, and about 15% (range, 2.3%-30%) of patients develop some degree of elevated serum TSH levels. This may develop in less than a month or may take several months. In about 5% of patients the clinical as well as laboratory indices are compatible with true myxedema.