What is the result of primary hyperthyroidism on TSH levels?

In the context of primary hyperthyroidism, the result on TSH (Thyroid Stimulating Hormone) levels is indeed undetectable levels. This condition is characterized by an overproduction of thyroid hormones (primarily T3 and T4) from the thyroid gland itself. As the levels of these hormones rise, they exert a negative feedback effect on the pituitary gland, leading to decreased secretion of TSH.

In a healthy feedback mechanism, when thyroid hormone levels are elevated, the hypothalamus reduces the release of Thyrotropin-Releasing Hormone (TRH), which in turn reduces the secretion of TSH from the anterior pituitary. This results in very low or undetectable levels of TSH in the bloodstream during hyperthyroidism, distinguishing it from secondary causes where TSH levels might be elevated due to a different deficiency.

Understanding this negative feedback loop is crucial for recognizing the biochemical changes that occur in hyperthyroid patients, and it highlights why monitoring TSH levels is a critical aspect of diagnosing and managing thyroid disorders.