As the name implies, prior to the onset of overt clinical symptoms, laboratory findings may provide a clue as to the development of a condition. Such is the case with subclinical hypothyroidism, in which the circulating levels of TSH are mildly elevated, but the thyroid hormones (eg, T3, T4) are still within the established reference intervals.
It may help to recall the mechanism of thyroid stimulation. The pituitary is attempting to release an increased amount of TSH to trigger the thyroid to produce more hormone, but from a laboratory standpoint, the measured amounts of thyroid hormone appear as well within the reference interval and are not shown to be decreased or in need of further stimulation. For this reason, TSH is often used as the front-line test to identify disorders of the thyroid.
Subclinical hypothyroidism may progress to overt hypothyroidism, but the timeframe varies from person to person. If the underlying cause is identified and remedied, the condition may revert to a euthyroid state. Some patients with subclinical hypothyroidism may dismiss some of the early, non-specific symptoms associated with hypothyroidism, such as inability to lose weight and constipation.