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How can iodine decrease the vascularity of the thyroid gland?

  1. May 26, 2013 #1

    If you give high doses of iodine thyroid hormone synthesis decreases. This means TSH level must increase. TSH function is to cause hyperplasia of the gland and increase vascularity of the gland. So how can iodine decrease vasculariy, when TSH is there to counter it. Thanks :smile:
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  3. May 26, 2013 #2

    Simon Bridge

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    TSH levels decrease with increased thyroxine levels - thyroxine is made from iodine, therefore, if low thyroxine levels was caused by iodine deficiency, increasing iodine consumption will decrease TSH ... also decreasing T4 production. It's a feedback process.
  4. May 26, 2013 #3
    Thanks for the reply Simon, but I still don't understand where I went wrong. I understand the feedback system

    1) I know iodine is needed for thyroxine production, but according to "wolf-chaikoff" effect, iodine in high doses actually deplete the production of thyroxine
    2) This means we give high doses of iodine, free T4 would decrease, this mean due to feedback TSH would increase.
    3) Now high TSH increases the vascularity of the gland. But high iodine concentration is given before thyroid surgery to decrease vascularity of the gland. Now how can high iodine do that in presence of high TSH?

    I get the feeling you know all this, and I may have misunderstood what you are saying. So please explain again where I went wrong. Thanks :smile:
  5. May 26, 2013 #4

    Simon Bridge

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    You are imagining a situation where the treatment would be counter productive.

    ... that would be if the patient has a too-high iodine level ... the feedback process remember?
    That is why increasing iodine in the diet is only useful if the patient has an iodine deficiency.

    Not all the iodine in the blood supply goes into T4 production.
    If iodine levels should drop, then the percentage that goes to T4 production needs to increase to maintain production to required levels. To do this, you have to increase iodine trapping, and the amount of blood that goes through the thyroid ... which is what THS is for. THS levels normally go up more than needed. Once production is back up, then THS levels fall to toughly the new level needed to maintain something like the proper T4 supply.

    But if there is a chronic shortage of iodine it does not matter how much THS is supplied, there is not enough iodine in the blood supply to make the required T4. Imagine an extreme case - no iodine available at all! The thyroid gets swollen - to no effect - and the patient also suffers the effects of not having enough T4 etc.

    Clearly the solution is to add more iodine - usually via the diet.

    Iodine levels increase, T4 gets made at a huge rate (because the body is geared to a bad shortage), when T4 levels reach requirement THS levels drop off, reducing T4 production to something more sustainable with the new level of iodine supply... the thyroid swelling goes down.

    As the iodine levels get higher than required, then iodine trapping is reduced so a smaller proportion of iodine gets supplied - the aim being to maintain T4 production by choking off the supply. However, it's not linear ... very high iodine levels can trigger an overreaction (so to speak) and T4 production drops too far for comfort.

    The normal healthy process will operate in a quasi-linear region where the feedback works sort-of smoothly.
    The process can get unhealthy for any number of reasons - so if you are being skeptical of claims that all thyroid problems can be addressed by iodine supplements then you are right to be so. It is possible to have too much iodine.

    Fortunately - "low iodine levels" is a likely cold-read, like "I feel there's someone with a C who has passed on..." and, for a healthy person, the supplements/remedies won't do much of anything ... cue the placebo effect and you have a pretty standard pseudoscience treatment.
    Last edited: May 26, 2013
  6. May 27, 2013 #5
    Thanks Simon again :smile: I think I got it this time, tell me if I misunderstood again.

    1) Ok so I give iodine, T4 production increases'
    2) I give lots of iodine, lots of T4 produced.
    3) This increase in T4, leads to feedback control and low level of TSH
    4) Now less TSH mean less proliferation and less vascularity--- purpose served
    5) Mean time high iodinee also inhibits organification of idoine, which is not related to the feedback system, but another protective measure

    I know it is not clear cut like this, but for basic understanding is this right

    My question then is if iodine is given before surgery to decrease vascularity and the way iodine does that is by increasing production of thyroxine, isn't that bad for patient who has hyperthyroidism wouldn't it exacerbate the symptoms. If u have hypothyroidism this seems like a good methods. But it makes me think since so many thyroid disorders mess up the feedback system, why is iodine given at all before surgery to reduce vascularity, it might have no effect at all. Also how long does it take for iodine to produce high levels of thyroxine to decrease TSH?
  7. May 27, 2013 #6

    Simon Bridge

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    Part of the action of THS is to increase iodine trapping.
    High T4 reduces THS which reduces iodine trapping (amongst other things).
    There are very probably other mechanisms.

    If the patient has an iodine deficiency, then THS levels are high and T4 levels are low ... giving the patient iodine is a good idea and there is overwhelming clinical evidence in favor.

    If the patient has an iodine surplus, then giving the patient iodine could make the situation worse.

    Before administering an iodine supplement, therefore, physicians should first check the patient's iodine levels.
    This is pretty routine in NZ. There are a lot of things that can go wrong and physicians have a duty to check that their treatment won't make things worse.

    I don't know the time-frames off the top of my head - you should consult the copious literature on the subject.
    Generally it varies depending on the details of the condition being treated.
  8. May 28, 2013 #7
    That's actually a pretty good question. High TSH levels do increase thyroid vascularity and can lead to goiter in the case of iodine deficiency. This, of course, is reversed by supplying iodine. With excess iodine the effect seems to be the inhibition of pituitary type 2 deiodinase (D2). Since D2 inhibits TSH, TSH levels rise but this is a pathological situation because the pituitary normally suppresses TSH levels when iodine and thyroxin levels are normal. So excess iodine seems to induce a type of hypothyroidism. The normal feedback mechanism between TSH and thyroxin levels seems to break down when pituitary D2 is suppressed by high iodine levels.


    Last edited: May 28, 2013
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