One flagged result, no explanation

A thyroid-stimulating hormone (TSH) result crosses the analyzer at 08:20 and releases at 42.6 mIU/L, well above this laboratory's reference interval of 0.40 to 4.50 mIU/L. The order did not include a free T4. The ordering clinic wants to know what the flag means before the next specimen is drawn. On its own, a high TSH could mean the thyroid gland has failed and is no longer making enough hormone, or it could mean almost nothing yet, because TSH is only half of a pair.

TSH is a stimulating hormone. It is made by the pituitary gland and its job is to drive the thyroid gland to produce thyroid hormone, chiefly thyroxine (T4). A single stimulating-hormone value tells a reader that the pituitary is signaling at a certain intensity, not why. The missing half of the story is the target-gland hormone, in this case free T4, because the same TSH value can mean different things depending on what free T4 is doing at the same moment.

Reading a stimulating hormone and its target hormone as one paired signal, rather than as two separate numbers, is the reasoning a bench scientist applies here. What the pattern of a stimulating hormone and a target-gland hormone can actually localize, and what it takes before that pattern is trustworthy enough to act on, is the question raised by this flagged TSH.

A stimulating-hormone result released without its paired target-gland hormone is an incomplete signal, and a reader should ask for or expect the pair before drawing a conclusion about where in the axis a problem sits.