Subclinical hyperthyroidism is a diagnosis that has emerged as a result of the increasing sensitivity of blood tests used to measure TSH levels. When these blood tests were first introduced for use in practice, they were primarily used to detect elevated TSH levels. The original versions of these blood tests were not sensitive enough to distinguish normal TSH levels from suppressed TSH levels. With further refinement, succeeding generations of blood tests were able to provide much more sensitive measurements of TSH levels. The precision of measurement gradually increased to the point where second and third-generation blood tests could be used to distinguish normal TSH levels from suppressed TSH levels. With widespread use of second and third-generation blood tests, it became apparent that there are some patients with suppressed TSH levels who actually have normal thyroid hormone levels. Strictly speaking, these patients do not meet the criteria necessary to make a diagnosis of thyrotoxicosis, as their thyroid hormone levels fall within the limits of reference ranges.
Based on what is known about the mechanisms that control interactions between the pituitary gland and the thyroid gland, it is believed that while these patients' thyroid hormone levels may appear to be normal in comparison to the rest of the population, in truth their levels are elevated above the limit of what their systems require to function normally. This interpretation is based on idea that the amount of TSH secreted by the pituitary gland provides the most sensitive indication of whether there is an adequate amount of thyroid hormone in the body. This diagnosis is more correctly referred to as subclinical thyrotoxicosis, though most doctors use the term subclinical hyperthyroidism in practice. Subclinical thyrotoxicosis may be caused by any of the same types of disorders that lead to overt thyrotoxicosis. The term "subclinical" refers to the fact that this diagnosis is based on the interpretation of blood tests. It does not necessarily indicate whether patients who present with this diagnosis have developed any symptoms or signs of thyrotoxicosis. The question of whether patients who present with subclinical thyrotoxicosis should undergo further evaluation and treatment is still a matter of debate.