The diagnosis of thyroid disease now depends as much on laboratory results as it does on clinical findings. One might therefore use the term “laboratory hyperthyroidism” when considering the spectrum of test results in thyrotoxicosis in the same manner that one employs the term “clinical hyperthyroidism” when evaluating patient signs and symptoms. Laboratory diagnosis is usually based on elevated values of serum T4 and T3, the two active thyroid hormones. Laboratory hyperthyroidism can be subdivided into three categories, depending on T4 and T3-RIA results:

1. Standard T4/T3 toxicosis (both T4 and T3-RIA elevated).
2. T3 toxicosis (T3-RIA elevated, T4 not elevated).
3. T4 toxicosis (T4 elevated, T3-RIA not elevated).

Standard T4/T3 toxicosis includes nearly 95% of cases of hyperthyroidism.

T3 toxicosis is estimated to occur in 3%-5% of hyperthyroidism patients (literature range, 2.4%-30%). Although T3-RIA values are increased in T3 toxicosis, the T4, THBR, and RAIU values are usually all normal. There is some evidence that T3 toxicosis is more common in patients with atrial fibrillation, and it may be associated more often with Plummer’s disease than with Graves’ disease (although not all investigators agree). One report indicates that T3 toxicosis is more common in iodine-deficiency areas (although some of these cases might have been pseudo-T3 toxicosis with T4 levels decreased because of the iodine deficiency). TSH is decreased in T3 toxicosis.

T4 toxicosis has not received much attention, and it was assumed to be less frequent than T3 toxicosis. In the more recent articles on this subject in the literature, incidence ranged from 0%-21% of hyperthyroid patients, although the true incidence is probably less than that of T3 toxicosis. Several investigators report that T4 toxicosis is more commonly found with iodine-induced hyperthyroidism (Jod-Basedow disease). T4 toxicosis is also found more commonly in elderly persons, but in some reports it is difficult to be certain that some cases were not actually T4/T3 toxicosis with depressed T3-RIA values due to concurrent severe illness.