TSH

Where it comes from

Made in

Thyrotroph cells of the anterior pituitary

Sent out by

TRH drives it, thyroid hormone feeds back on it in a tight loop; a night-time rise

Acts on

• Thyroid (iodine uptake, hormone synthesis and release, gland growth)
• Bone and fat, weakly

What it does

• Sets how much thyroid hormone is made
• Rises when the thyroid fails and falls when it overproduces, so it is the first test for either
• In Graves’ disease an antibody mimics it and the gland runs unchecked

In medicine

• The thyroid screening test: high in hypothyroidism, suppressed in hyperthyroidism, and the number levothyroxine doses are adjusted to
• Thyrotropin alfa, recombinant TSH, prepares thyroid cancer patients for radioiodine and scanning without stopping their thyroxine ³

Beyond the body’s own use

• What it settles: because a small change in thyroid hormone moves TSH a long way, it is the most sensitive single test of how the thyroid is doing
• What it cannot settle: in pituitary or hypothalamic disease TSH may be normal or low while the thyroid is underactive, so it has to be read with free thyroxine rather than alone
• What can mislead it: biotin supplements, heterophile and anti-ruthenium antibodies, and macro-TSH bound to immunoglobulin all give results that do not match the patient, and a result that does not fit should be repeated by another method ²
• What treating a borderline result achieves: in 737 adults over 65 with mildly raised TSH, levothyroxine changed neither tiredness nor hypothyroid symptoms over a year ¹