I am bemused and confuddled. Finally got a printed copy of my blood test results. Thyrotropin level is very low. But free T3 and free T4 are in the normal range. My understanding is that low thyrotropin is indicative of hyperthyroid state. So why are the free T3 and T4 normal. At least one should be high, shouldn’t it?

What is very low? There’s quite a range…

Very low – 0.006.

Previous readings have been2.19, 1.84, 1.52 over 20 years.

Oh, sorry, over 6 years. It wasn’t tested in the early tests.

buffy said:

I am bemused and confuddled. Finally got a printed copy of my blood test results. Thyrotropin level is very low. But free T3 and free T4 are in the normal range. My understanding is that low thyrotropin is indicative of hyperthyroid state. So why are the free T3 and T4 normal. At least one should be high, shouldn’t it?

(Checking Google)
“Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, TSH, or hTSH for human TSH) is a pituitary hormone that stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3) which stimulates the metabolism of almost every tissue in the body.”

“The concentration of thyroid hormones (T3 and T4) in the blood regulates the pituitary release of TSH; when T3 and T4 concentrations are low, the production of TSH is increased, and, conversely, when T3 and T4 concentrations are high, TSH production is decreased. This is an example of a negative feedback loop.”

So if T3 and T4 concentrations are high then TSH is low. But if TSH is low then T3 and T4 are low. Nice logical paradox here, but it’s not a real paradox, just a negative feedback loop.

So in Buffy’s case, T3 and T4 are normal but the body thinks they are high, so stops producing TSH, but the breakdown rate of T3 and T4 are so slow in Buffy’s body that the low TSH doesn’t affect levels. Or it’s a short term transient thing, shorter than the time delay of the negative feedback loop. Either way, the only one that really matters is T3.

Most stuff suggests T4 is the important one.

Anyway, I’m going with a possible lab error.

I’ll go and have another test in about a year.

That makes sense.

buffy said:

Most stuff suggests T4 is the important one.

Anyway, I’m going with a possible lab error.

I’ll go and have another test in about a year.

T4 is converted where required by the body to T3 when needed – T3 is the ‘active’ form, T4 is like T3 on a bus…

You have a mild case of hyperthyroidism.

Thanks poik.