What does a high TSH level indicate?

A high TSH (thyroid-stimulating hormone) level indicates an underactive thyroid, also known as hypothyroidism. TSH is produced by the pituitary gland, and its main role is to stimulate the thyroid gland to produce hormones, primarily thyroxine (T4) and triiodothyronine (T3). When the thyroid does not produce enough of these hormones, the pituitary gland responds by secreting more TSH in an attempt to boost hormone production. Thus, elevated TSH levels are a direct indication that the thyroid is not functioning adequately to meet the body's needs.

In contrast, if the thyroid were overactive (hyperthyroid), TSH levels would typically be low, as the body tries to reduce stimulation of the thyroid. Healthy or normal thyroid function is indicated by TSH levels within a specific reference range, where there isn't an excess or deficiency of production. Therefore, understanding TSH levels is crucial for diagnosing thyroid conditions accurately.