Is TSH stimulated or inhibited by T3 and T4?

Thyroid-stimulating hormone (TSH) is indeed inhibited by the levels of T3 (triiodothyronine) and T4 (thyroxine) in the bloodstream. This occurs through a negative feedback mechanism in the hypothalamic-pituitary-thyroid axis.

When the levels of T3 and T4 rise, they signal the hypothalamus and pituitary gland to reduce the production and release of TSH. Specifically, elevated thyroid hormones act on the pituitary gland to decrease its sensitivity to thyrotropin-releasing hormone (TRH), which is produced by the hypothalamus and stimulates the secretion of TSH. Additionally, high levels of circulating T3 and T4 can directly inhibit TSH synthesis and secretion from the pituitary gland.

This feedback mechanism plays a crucial role in maintaining hormone balance and ensuring that thyroid hormone levels remain within a healthy range, thus preventing excessive stimulation of the thyroid gland and potential hyperthyroid conditions.