What assay is used to confirm borderline cases of hypothyroidism?

The thyrotropin-releasing hormone (TRH) stimulation test is utilized to confirm borderline cases of hypothyroidism because it evaluates the functional status of the pituitary gland in response to TRH. In individuals with hypothyroidism, especially those with borderline TSH levels, the response to TRH is typically diminished or abnormal.

When TRH is administered, a healthy pituitary gland should produce increased levels of TSH (thyroid-stimulating hormone) in response. In patients with primary hypothyroidism, this response is altered, thereby providing crucial information on the status of the thyroid axis. This test is particularly useful when the TSH levels fall within the range that may be considered borderline, giving clinicians additional insight to support a definitive diagnosis of hypothyroidism.

Other assays, while important for overall thyroid function evaluation, do not provide the same confirmatory evidence as the TRH stimulation test for borderline cases specifically. For instance, assessing free T3 or free thyroxine levels might not fully clarify the function of the hypothalamic-pituitary-thyroid axis in such nuanced cases.