Thyroid Stimulating Hormone

Thyroid stimulating hormone, known as TSH, is produced by the pituitary gland and serves as the primary signal that tells your thyroid gland how much thyroid hormone to make. It is often the first marker checked when thyroid health is being assessed, because it is an exquisitely sensitive indicator of whether your thyroid is producing the right amount of hormones. A TSH level within the expected range usually means the communication between your pituitary gland and your thyroid is working normally.

Think of TSH as a thermostat in your home. When the room is too cold, meaning thyroid hormone levels are low, the thermostat turns the heating up by releasing more TSH, prompting your thyroid to produce more hormone. When the room is warm enough, the thermostat dials back. This feedback loop means that TSH often moves in the opposite direction to your thyroid hormones: if your thyroid is underperforming, TSH tends to rise as your pituitary gland tries to coax it into action, and if your thyroid is overproducing, TSH drops because the pituitary senses there is already plenty of hormone circulating. This is why TSH is such a powerful screening tool: even subtle shifts in thyroid function can show up as changes in TSH before the thyroid hormones themselves move outside the normal range.

Various factors can affect your TSH reading. It naturally fluctuates throughout the day, tending to be highest in the early morning and lowest in the afternoon, so the time of your blood test can influence the result. Stress, illness, significant changes in weight, and certain medications, including biotin supplements, steroids, and some heart medications, can also shift your TSH. Age is another consideration, as TSH reference ranges can differ slightly for older adults. If your TSH level is outside the expected range, your GP will typically look at your free T4, and sometimes free T3, alongside it to build a clearer picture of your thyroid health and discuss any next steps with you.