HormonesmIU/l
TSH (thyroid)
Optimal
0.5–2.5 mIU/l when young; 1–4 is normal above 70
Why
The metabolic “thermostat”. Mild underactivity (TSH 4–10) causes fatigue, weight gain and high LDL, and affects 10% of women over 40 (Hashimoto's). Note: centenarians' TSH is slightly HIGHER — a “slower thyroid” in old age may be protective, so not everyone needs treatment.
How often
Every five years from 35; yearly with symptoms, pregnancy or autoimmune disease, with anti-TPO.
What moves it
Correcting iodine and selenium, levothyroxine (if TSH > 10 or symptomatic above 4); biotin supplements falsify the assay — stop 3 days before testing.
Last reviewed: 6 September 2026
This site is educational and does not replace medical advice. Start any supplement or drug only after consulting a doctor or pharmacist.