TSH

What is TSH?

TSH is the hormone your brain uses to drive your thyroid. A high value often means the thyroid needs a nudge; a low one, the opposite. Your thyroid regulates your energy, weight and mood. TSH is the first, sensitive signal of whether that control is in balance.

Clinical frame

TSH is the most sensitive first-line test of thyroid function: because of the log-linear feedback, a small fall in free T4 causes a much larger rise in TSH, so TSH deviates before free T4 does. Free T4 is only measured once TSH falls outside the reference interval ^[1].

An elevated TSH with a normal free T4 is called subclinical hypothyroidism and occurs in up to 10% of adults. The risk of heart failure and coronary death rises mainly from a TSH of 10 mIU/L. Mildly elevated values mainly call for follow-up, and in older adults the upper limit is physiologically higher ^[2].

A suppressed TSH with a normal free T4 points to subclinical hyperthyroidism, with more atrial fibrillation, bone loss and fractures, most clearly at a TSH below 0.1 mIU/L and above the age of 65 ^[3]. Because reference intervals differ between assays and TSH can fluctuate temporarily, a repeat measurement confirms the finding ^[1].

What moves this value?

The thyroid picture is nuanced and moves with heredity, iodine, stress and sometimes autoimmune processes.

This is mainly a marker to follow, not to adjust yourself. Take an unusual value to your doctor as a talking point.

What this connects

• FT4

References

1. Van Uytfanghe, K. et al. Thyroid stimulating hormone and thyroid hormones (triiodothyronine and thyroxine): an American Thyroid Association-commissioned review of current clinical and laboratory status. Thyroid 33, 1013–1028 (2023). doi.org/10.1089/thy.2023.0169
2. Biondi, B., Cappola, A. R. & Cooper, D. S. Subclinical hypothyroidism: a review. JAMA 322, 153–160 (2019). doi.org/10.1001/jama.2019.9052
3. Biondi, B. & Cooper, D. S. Subclinical hyperthyroidism. N Engl J Med 378, 2411–2419 (2018). doi.org/10.1056/NEJMcp1709318