Question: If free T3 looks good, why is TSH still a little high? Why hasn't the T3 brought it down enough?
Your thyroid gland makes thyroid hormone. Thyroid hormone increases your metabolic rate and does a lot of related things. Your hypothalamus is governing that by controlling your pituitary, the master endocrine gland, and its secretion of TSH, which is what controls the thyroid gland and makes it make more thyroid hormone.
The way that the feedback occurs is that the circulating T4 is converted to T3 inside the cells of the pituitary. That is what suppresses the production of TSH, which is basically the pituitary monitoring the thyroid hormone levels to know whether the thyroid has done its job. If the pituitary, the master endocrine gland, decides that the thyroid has done its job, it takes down TSH, the signal to make more thyroid hormone.
You really are not looking at whether the free T3 is suppressing the TSH. Ninety percent of that suppression comes from circulating T4 that's converted to T3 inside the pituitary gland. You really are looking at whether the T4 is on the high end of normal or not.
If your reverse T3 is on the higher end of normal, then that explains it. You basically have your brain telling your thyroid gland that it needs more thyroid hormone, but you have much of the rest of your body deciding that it's not in the position to carry out the effects on the metabolic rate that the thyroid hormone is demanding. It's converting the thyroid hormone into reverse T3, which is basically a thyroid antagonist.
If your reverse T3 is high, then I think you want to look at things like calorie intake, carb intake, and stress levels because I think those are the main things that might make your body want to resist the signal of thyroid hormone by making the reverse T3. If the reverse T3 is good, meaning it's pretty low, then I think that means that there is something either in your brain, specifically in the hypothalamus or in the pituitary or somewhere in the combination where they're just deciding that your body needs more thyroid hormone than you have.
My suspicion is that that's going to relate to how sensitive your cells are to the thyroid hormone, if your cells are somewhat resistant. Remember in the last AMA, this got brought up, and I talked about zinc deficiency and high free fatty acids being the primary things that are going to reduce sensitivity to thyroid hormone or cellular uptake. There are some indications that high free fatty acids might also decrease cellular uptake, but not much is known about what governs cellular uptake.
In fact, there are some genetic variations in cellular uptake. If the thyroid hormone levels are high in your blood because they're not getting into the cells, then that could easily explain everything. It's just that your problem seems pretty moderate because you're not saying that your thyroid hormones are sky high and your TSH is sky high. You're just saying everything is a little on the high side of normal.
It sounds like there's not a big problem, but that something somewhere your body is determining that you need a little bit more thyroid hormone. If you can address zinc, free fatty acids, and I would address zinc and free fatty acids as the top things, unless the reverse T3 is high, target carbs, calories, and stress.
This Q&A can also be found as part of a much longer episode, here: https://chrismasterjohnphd.com/podcast/2019/03/08/ask-anything-nutrition-feb-23-2019
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