In which condition is serum TSH level almost absent?

In primary hyperthyroidism, the serum TSH (thyroid-stimulating hormone) levels are almost absent due to the negative feedback mechanism involved in thyroid hormone regulation. In this condition, the thyroid gland is overactive and produces excessive amounts of thyroid hormones (T3 and T4).

As the levels of these hormones rise, they exert a strong negative feedback on the pituitary gland, leading to a significant reduction in TSH secretion. Since TSH is responsible for stimulating the thyroid gland to produce hormones, its low levels reflect the high concentration of thyroid hormones already present in the bloodstream. This dynamic is a key feature that helps differentiate primary hyperthyroidism from other thyroid and metabolic disorders.

In contrast, conditions like primary hypothyroidism would typically show elevated TSH levels due to the lack of sufficient thyroid hormones prompting the pituitary to produce more TSH. Euthyroid sick syndrome is characterized by variable TSH levels due to non-thyroidal illness, and secondary hyperthyroidism would exhibit normal or elevated TSH levels because the issue lies in the hypersecretion of TSH itself, often due to a pituitary adenoma or other stimulating factors.