You are welcome to print this out and take it to your doctor.  I wrote this originally on https://healthunlocked.com/thyroiduk

There’s no way of knowing if your TSH is “fine” or “normal” without knowing what your FT3 and FT4 are.  TSH on its own tells you nothing.

A quick lesson in thyroid hormones (very simplified). Your pituitary produces TSH (thyroid stimulating hormone). This does what it says on the tin – it stimulates your thyroid to produce 5 things, including T4 and T3.

T4 is an inactive hormone which is converted by the body into T3. T3 is the active hormone which is used by every cell in the body.

It is lack of T3 that makes you hypo and too much t3 that makes you hyper. This is why it is essential to measure FT3, that is the only reliable indicator of whether or not you are hypo.

You need to know FT4 also as you need to know if you are converting effectively from T4 to T3. If both are low in range, then you need more thyroxine (T4). If FT4 is high in range but FT3 low, then you are not converting properly and might benefit from the addition of some T3.

TSH is only useful once a patient has been diagnose as hypo. If it is high, then your pituitary is working properly and you have primary hypothyroidism.

If you are hypo (FT3 low) and TSH is low then your hypothyroidism is secondary (or tertiary) – you have an under performing pituitary or hypothalamus.

You can also refer your doctor to the Pulse online Continuing Professional Development module here .

This module clearly states that secondary hypothyroidism is characterised by TSH low/low normal and FT4 low.  Yet few doctors will even test FT4 if TSH is low, so most can never diagnose secondary hypothyroidism.Diagnosing secondary hypothyroidism