In my textbook it is stated that the T3 hormone produced at the thyroid gland is 3-5 times more biologically active than T4.

How is being active defined and how can this effect be measured?

  • $\begingroup$ This is quite a general question (and bit too broad). Activity of bioactive molecules such as hormones can be defined as how much change do the make to the receipient cells. For eg. let's take two growth hormones A and B. If they are given to some cells at equal concentration but hormone A induces proliferation 5 times more frequently than hormone B, then A is 5 times more active than B. This is not the best definition / example but I think you can get the point from this. $\endgroup$ Commented Apr 14, 2015 at 13:57
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    $\begingroup$ This question is not broad and pretty well focused. I did a literature search and it is actually hard to find! In all a good question from my point of view +1. $\endgroup$
    – AliceD
    Commented Apr 14, 2015 at 14:46
  • $\begingroup$ @AliceD - I have to disagree with you on this one. I think we both agree that the effect of a hormone or any biologically active molecule can be measure in several ways, eg gene expression by RT PCR, flurescent riporter gene, riporter gene activity assay etc. and that's just one thing and few examples. This in my view is too broad. But on the other hand I might interpret the question in the wrong way.... $\endgroup$ Commented Apr 14, 2015 at 18:37
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    $\begingroup$ If you think about a jackhammer, the better effector is the one that can chip away the most concrete. If the jackhammer had low power or no power (T4), we can safely imagine you'd have lower efficacy than the full-power jackhammer (T3). We can measure the activity of a jackhammer as a function of the work done vs. power provided. In much the same way, we can measure biological activity as a function of concentration vs. catalytic/binding activity. All they're saying is that T3 has more activity at comparable concentrations to T4. Plasmon surface resonance can help measure binding/interactions. $\endgroup$
    – CKM
    Commented Apr 14, 2015 at 21:37

3 Answers 3


Activity of a hormone represents the efficacy of the hormone in causing the intended effect.

In case of T3 vs T4, though T4 is the majority fraction, T3 is more active, meaning it is much more efficacious in effecting the results.

The concentration of T4 is 30 - 50 times higher than T31. Though T3 is produced by thyroid it is in minority. The majority of T3 is produced by peripheral conversion of T4 into T3.

Thyroid hormones are transported in the blood vessels by albumin, thyroxine binding prealbumin (also called transthyretin). T4 binds more tightly to these transport proteins than T3. In other words T3 constitutes the majority of the free fraction (unbound to carrier/transport proteins). Only the free form of the hormone is metabolically active. So T3 is 3 to 5 times more active than T4.

T3 - triiodothyroxine

T4 - tetraiodothyroxine

1: pg. 312, Essentials of surgery, Sanjay Azad


Biological activity has a range of definitions, but in this case I believe it to mean that the T3 hormone is produced at a rate 3-5x that of T4. This can be measured with fluorescent tags. Typically, fluorescent tagging uses a reactive derivative of a fluorescent molecule, called a fluorophore. The fluorophore selectively binds to a specific region or functional group on the target molecule and can be attached chemically or biologically. Using technologies, such as microscopes, fluorescence can be viewed/imaged; this allows scientists to count, track, and/or monitor molecules over time.

  • $\begingroup$ Thanks for answering! Could you back these figures up with a reference or two? $\endgroup$
    – AliceD
    Commented Apr 14, 2015 at 20:49
  • $\begingroup$ It is unclear what you mean by "produced at rate 3-5x of T4" because 90% of all hormonal output from thyroid is T4, but T4 later get converted to T3 at liver and kidney. Do you mean the net total T3 present in the body at any given time? The sadly text is also unclear what it means by "biologically active", it is as if T3 has little legs and sprints while T4 crawls, not a very helpful image $\endgroup$
    – Fraïssé
    Commented Apr 15, 2015 at 4:30

In my opinion, activeness of a hormone is more about its effect than its production rate. Cortisol and Aldosterone are both adrenal hormones, yet as a mineralocorticid, aldosterone is the active one by great margin (due to intracellular degradation of the glucocorticoids).


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