- Thyroid-stimulating hormone (TSH), also known as thyrotropin, is a critical glycoprotein hormone secreted by the anterior pituitary gland. Its primary role is to regulate the function of the thyroid gland, which produces the hormones thyroxine (T4) and triiodothyronine (T3).
- These thyroid hormones control key metabolic processes, including energy expenditure, protein synthesis, thermogenesis, and growth and development. TSH serves as the principal regulator of thyroid hormone production, making it a cornerstone of the hypothalamic-pituitary-thyroid (HPT) axis.
- The secretion of TSH is controlled by thyrotropin-releasing hormone (TRH), which is produced in the hypothalamus and delivered to the anterior pituitary through the hypophyseal portal system. In response to TRH, the anterior pituitary releases TSH into the bloodstream. TSH then travels to the thyroid gland, where it binds to TSH receptors on the surface of thyroid follicular cells. This binding stimulates a cascade of biochemical events that lead to increased uptake of iodine, synthesis of thyroglobulin, and secretion of T3 and T4.
- Once T3 and T4 are released into the circulation, they exert negative feedback on both the pituitary and hypothalamus to regulate further TSH and TRH secretion. When circulating thyroid hormone levels are sufficient or elevated, TSH production decreases. Conversely, when T3 and T4 levels fall, TSH secretion increases to stimulate thyroid activity. This tightly controlled feedback loop ensures hormonal balance and stable metabolic function.
- TSH has a diurnal rhythm, with peak levels typically occurring during the night and early morning hours. While its physiological role is mainly associated with thyroid function, TSH also affects other biological processes. For example, it influences thyroid gland growth, and excessive TSH stimulation—such as in iodine deficiency—can result in goiter, or thyroid gland enlargement.
- Clinically, TSH is a central biomarker in the diagnosis and management of thyroid disorders. High TSH levels usually indicate hypothyroidism, where the thyroid is underactive and not producing enough hormones. In contrast, low TSH levels often suggest hyperthyroidism, in which excessive thyroid hormone suppresses TSH release. Measuring TSH is often the first-line test in thyroid function panels, sometimes alongside free T4 and T3, to gain a comprehensive understanding of a patient’s thyroid status.
- Abnormal TSH levels can result from primary thyroid diseases or from secondary causes related to pituitary or hypothalamic dysfunction. For instance, in secondary hypothyroidism, the pituitary fails to produce adequate TSH despite low thyroid hormone levels. TSH levels can also be influenced by stress, illness, medications (like corticosteroids or dopamine), and pregnancy, where physiological changes often alter thyroid hormone dynamics.
- In therapeutic contexts, synthetic TSH is used in certain diagnostic procedures, such as monitoring thyroid cancer recurrence, and as part of radioactive iodine uptake tests. TSH suppression therapy, in which TSH levels are intentionally kept low, is also employed in some patients with a history of thyroid cancer to minimize stimulation of residual cancerous tissue.
