Half Lives of T3/T4

The biological half-life of a substance such as a drug, hormone, or metabolite, is the time it takes for that substance to lose half of its pharmacologic or physiologic activity.   For example, the half life of T3 is short, typically about 4-6 hours in most people.  What this means, is that somewhere between 4-6 hours after taking T3, only about 50% of the original dose of the drug will be left in the body.  The other 50% has already been metabolized off, or “used” by the cells.  Another 4-6 hours after that, another 50% of the remaining T3 will be metabolized off or “used up” again, leaving only 25% of the original dose in the body.  And so forth.  So you can see that when a dose of T3 is taken, the full effect of thatT3 will really only last about 4-6 hours, before it basically starts wearing off.    However, a single dose of T3 will last about 24 hours in the body more or less, depending on an individual’s metabolism of it (6 hour half life X 4 half life cycles = about 6.25% left in the body after 24 hours).   The same thing is true, by the way, of the endogenous T3 your thyroid gland makes.  It also has a short half life, exactly the same as the medication does.   You’re going to get the greatest benefit – as well as the worst side effects – in that first 6 hours or so, and providing no more T3 is produced, those effects will slowly wear off over the next day or so.

Why is the half life of T3 so short?  T3 is considered the metabolically active hormone.  In other words, it is T3 that for the most part is used by cells and mitochondria to give us energy, and is used for numerous other cellular processes as well.  T4 is often considered the “storage” hormone, because it’s far less metabolically active than T3.  In fact, most of our T3 is made from T4.  Think of T4 like the gas in the tank, and T3 is the energy you get when you put your foot on the accelerator.  Because T3 is so potent, it’s “in and out” of our cells quickly on an “as needed” basis.  It’s the “ON” button for our cells and mitochondria, and it’s important that the effects don’t last long as a result.  When people are suffering from hyperthyroid symptoms, it’s often because the T3 level is very high all the time, as if the gas pedal is floored all the time, or the “on” button can’t turn off.  And as anyone who has experienced hyperthyroidism can tell you, it can be a pretty unpleasant experience all around, ranging from the mildly unpleasant (anxiety, tremors, sweating, heart palps, headaches) to just plain dangerous (mania, hallucinations, A-fib or other arrhythmias, life threatening tachycardias, and death).

The good news is, if you take too much of a single dose of T3 and feel some “hyperthyroid symptoms” because of that, typically these effects will start wearing off within a few hours.  Depending on how much T3 you took, and your own personal metabolism, it might be longer, but in general, within a day or two, the total effects of taking one large dose of T3 will pretty much wear off.

The bad news is, if you want to keep a constant level (or “steady state”) of T3 in your system, you have to keep taking it, to prevent feeling the “T3 wobble” – the ups and downs of taking a very “short acting” medication.  This is why people who take T3, either alone or in Armour or Naturethroid, usually do better taking T3 at least twice a day, and three times a day is even better.  There are also some “longer acting” T3 medications on the market to attempt to address this problem.  That way, you tend to get a more “even” or consistent amount of T3 in your blood stream.

People have varying metabolism rates, so T3, like any drug, can “build up” in your system if you take too much of it and don’t allow enough time for the previous dose to “wear off”.  So people who take T3 usually learn through trial and error how much and how often of a certain dose of T3 they need to feel good overall.

The half life of T4 is very different than the half life of T3.  It is much longer – usually about 7-10 days in people.   Note how this is days, not hours, like T3.  This can be shorter or longer, of course, depending on everyone’s individual metabolism.   What this means is that the effects of T4 just aren’t felt very much one way or the other until about 7-10 days have passed.  In other words, if suddenly something came along and wiped out all your T4 for example, and your thyroid gland wasn’t making more to make up for this, the soonest most people will really start to feel the effects of this is about 7-10 days.  Remember, T4 is not as metabolically active as T3 is either, so it will probably be yet another 7-10 days (14-20 days from the original insult) before you really start feeling the effects of decreased T4.   Since most T3 is made from T4, keep in mind that T3 will be decreasing as well.  By 21-30 days post insult, you’d really be feeling it though.  Just ask any thyroid patient who has had to discontinue their T4 meds cold turkey in preparation for a thyroid uptake and scan test —  some people are ready to commit suicide by the 3-4 week mark.  It’s so horrific for people to feel this acute drop in T4 (and T3 levels which mostly come from T4), that patients who had experienced it once never wanted to go through it again.  So the medical profession had to come up with what they call “compassionate alternatives” for those who needed repeat testing, because patients refused to do it.  It turns out that suddenly depriving someone of all thyroid hormone is an incredibly “un-compassionate” thing to do.

The opposite is true as well when it comes to T4.  If a severely hypothyroid person starts taking T4 medication, it’s generally about 3-6 weeks before they start feeling better.  This is also because of this long half life of T4.  It takes as long to “build up and accumulate” to a steady state level as it does to dissipate.

Iodine typically has a half life of about 4-6 hours in people, just like T3.  So just like T3, if you take one dose of iodine, the full effects of that will last about 4-6 hours in general in most people, and then slowly dissipate after that.

So, why do I think its important to know about the different half lives of T3 and T4?  Hopefully, you’re beginning to get the idea that when it comes to thyroid hormones and iodine, there are some rather predictable time frames for symptoms that might develop as a result of deficiencies or excess of T4, T3, and Iodine.  And these time frames in part are based on the half lives of each.  I already gave one example above of what might happen if both T4 and T3 were wiped out suddenly, with no replacement being produced.  A T3 deficiency would be felt first, within days.  The T4 deficiency would develop over time, probably peaking in the 3-6 week timeframe, with all the other symptoms of severe hypothyroidism developing.  The opposite could occur as well:  if a thyrotoxic insult occurred which caused a sudden and extreme thyrotoxicosis (sudden and severe release of thyroid hormones from the thyroid gland), the immediate effects of this would be due to the sudden increase in T3 throughout the body.  The longer term effects would be due to the increase in T4.  If this sudden thyrotoxicosis were just a one time event, the T3 effects might be felt and then wear off, and the longer term T4 effects might be very mild, perhaps not even noticeable.  However, with a continued thyrotoxicosis, over several days or weeks, there will not only be the immediate effects of T3, but the longer term effects of T4 as well as T3 continued accumulation will be also.

When I was ON thyroid hormone medication, I felt the effects of taking T3 the most.  I could control these symptoms the most because of the short half life of T3.  If I got a little too “high” or “low”, I knew it would just be a matter of a day or two to correct it.  With T4, it was more difficult to control the correct dosage sometimes, because the consequences of changing the medication dose weren’t fully felt until weeks later.  Iodine acted just like T3 in my non-thyroidal cells and had the same half life, so that, too was a little easier to control than T4.   But one thing I always had to keep in mind with all this, was that there were always “shorter T3 and iodine cycles” going on within the “longer T4 cycle”.

Understanding these half life cycles helped me to figure out if my symptoms were being caused by too much Iodine or T3.  Knowing this information allowed me to adjust my diet or medication accordingly.   It also helped me to “ride out” these symptoms, because I could predict how long it would be before they would dissipate.   The half lives helped determine how long I was going to feel symptoms from flaring of endogenous T3/T4 and Iodine as well.  There were more immediate, but shorter lived consequences for T3, and then the delayed and/or longer term consequences of T4 due to flares.  Longer term consequences of flaring were going to depend on the duration and severity of the flaring overall.  Thankfully, I was able to tolerate NSAID’s well post floxing.  Although I avoided them as much as possible, when I was desperate, they would very much help to stop the flaring of my thyroid gland, and then it would be a matter of waiting for the symptoms to subside.

So some key points to ponder from this section would be:

  1. It was the half lives of T3 and T4 medication, and ingested Iodine which determined how long I would feel symptoms after I took dosages or ingested them. I would experience one set of symptoms when I first took T3 and Iodine, and then another set of symptoms when they’d wear off. The same was true of T4, but with a longer, delayed cycle, and possibly milder symptoms because T4 is not as metabolically active.
  1. It was the half lives of endogenous T3, T4 and Iodine which determined flares and how long I was going to feel them. There were more immediate consequences for T3 in a flare, and delayed and/or longer term consequences for T4 in a flare.
  1. I believe that my acute and severe March 2010 Cipro floxing included a thyrotoxic event(s). There were more immediate consequences for the endogenous T3 change, and longer term consequences for the endogenous T4 change. The thyroid gland will be trying to produce thyroid hormone if it senses a large drop, or stop production if there’s a large increase. In my case, I have Hashi’s, either brought on or exacerbated by my first exposure to FQ’s. Unknown to me, I had a “struggling” thyroid gland already from the first floxing, and I think my gland just couldn’t respond well anymore. So even while the flaring was occurring, little or no real thyroid hormone production was occurring. This had consequences for my symptom development both acutely in the shorter term, and delayed in the longer term.


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