Elevated thyroid hormone levels suppress pituitary secretion of thyroid-stimulating hormone (TSH) through a negative feedback mechanism.

Study: Thyroid-Stimulating Hormone (TSH)-Secreting Pituitary Tumor Misdiagnosed for 20 Years: Possible Effect of Long-Term Treatment With Thyroid Hormone

Even when someone takes a lot of thyroid hormone pills, their body still tries to slow down TSH production — and this case shows that even a rare tumor making TSH didn’t grow much, suggesting the body’s feedback system still worked a little.

Study: An Abnormality of Thyroid Hormone Receptor Expression May Explain Abnormal Thyrotropin Production in Thyrotropin-Secreting Pituitary Tumors

When thyroid hormone levels go up, the brain normally tells the pituitary to stop making TSH — like turning off a faucet. This study found that in some tumors, the pituitary can't 'hear' the thyroid hormone signal because it's missing the right receptors, which proves those receptors are needed for the signal to work.

Study: Familial inappropriate TSH secretion: evidence suggesting a dissociated pituitary resistance to T3 and T4

When thyroid hormone levels go up, the brain usually tells the pituitary to make less TSH — this study shows that still happens with one type of thyroid hormone (T3), even when another (T4) doesn’t work right in some rare cases.

Study: Sustained pituitary T3 production explains the T4-mediated TSH feedback mechanism.

When there's more thyroid hormone in the blood, the pituitary gland converts some of it into a more active form that tells the pituitary to make less TSH — like a thermostat turning down the heat when the room gets too warm.