Thyroid Tests Explained: T3, T4, TSH and What the Numbers Mean
Thyroid imbalance affects weight, energy, mood, hair and menstrual cycles — and most people only find out through a blood test. The good news is that the test is simple, inexpensive and can be collected from home.
Here is what T3, T4 and TSH actually measure, and how to read the report without panicking.
What each value means
TSH comes from the pituitary gland in the brain and acts as the thermostat. T3 and T4 are the hormones your thyroid actually produces.
In hypothyroidism, TSH is high because the brain is shouting at a thyroid that is not responding. In hyperthyroidism, TSH is usually very low because the thyroid is overproducing on its own.
- High TSH + low T4 — hypothyroidism (underactive thyroid)
- Low TSH + high T3/T4 — hyperthyroidism (overactive thyroid)
- TSH slightly outside range — often re-tested before any medicine
When should you give the sample?
Morning is preferred. If you take thyroid medication, doctors usually advise giving the sample before your daily dose — the exact timing should be consistent between tests, otherwise the comparison is meaningless.
How to take thyroid medicine correctly
Levothyroxine is absorbed best on an empty stomach, 30 to 60 minutes before breakfast, with water only. Coffee, milk, calcium and iron supplements block absorption significantly — keep a gap of at least four hours between these and your thyroid tablet.
Consistency matters more than the exact clock time. Taking it at the same time daily is what keeps levels stable.
When to re-test after a dose change
After any change in dose, re-test in 6 to 8 weeks. Levels need that long to stabilise, and testing sooner gives a number that reflects neither the old dose nor the new one.
Once stable, an annual test is usually enough.
Do not self-prescribe thyroid medicine
This is more common than it should be. Someone shares their thyroxine strip, or a chemist supplies it without a prescription. Taking thyroid hormone when you do not need it causes weight loss, palpitations, anxiety and bone thinning — and it takes a doctor to notice the cause.