Why You Have Hypothyroid Symptoms When Your TSH Is Normal (Or You're Taking Synthroid)

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Ok.  I’m about to totally geek out about thyroid hormones – so consider this your warning!  Control of production and activation of thyroid hormone is really poorly understood by both physicians and patients alike.  As your best advocate for your own overall health, I think it’s important for patients to take an active interest in learning about these things.  If you suffer from infertility, recurrent pregnancy loss, depression, unintentional weight gain, constipation, cold body temperature, hair loss, dry skin, brittle fingernails, fatigue, or have been diagnosed with hypothyroidism – please, please, please take the time to read this article.  Alright, enough begging ;)  On to the goods.




This is a pretty common scenario: a patient comes in to my practice, having been identified in the past that they have a low functioning thyroid (“hypothyroidism”); they’ve been taking Synthroid (synthetic T4), and their TSH looks “normal,” but they still have a number of hypothyroid symptoms. 




“Looks like the medication is working… So how am I still having thyroid symptoms?? “ It’s more common than you’d think, actually.




Hypothyroid symptoms can range from constipation, to cold body temperature, depression, hair loss, dry skin, brittle fingernails, weight gain, fatigue, and infertility or recurrent pregnancy loss.  Lots of patients (and their docs) believe that a TSH is in the normal range means their thyroid is functioning well.  Unfortunately, in many cases this simply isn’t true.




Control of production and activation of thyroid hormone is a more complex process than it seems on the surface.  I educate patients about this process all the time, because naturopathic medicine recognizes that there are many things that affect thyroid hormone levels and activity in the body, and until we unravel where the problem is, a patient will continue to suffer the ill effects of a low thyroid.




The Basics




Hormonal control of thyroid hormone production starts in the hypothalamus (brain region) with the production of TRH.  TRH acts on the pituitary, which produces TSH.  TSH then speaks to the thyroid, requesting thyroid hormone production.




When TSH is released from the pituitary, the thyroid responds by producing thyroid hormones T3 and T4.  Approximately 97% of the hormone produced by the thyroid is in the form of T4, a relatively inactive thyroid hormone in the body.  T4 gets released into the body, and at many sites (the liver, the brain, and other organs) is converted into T3, the active hormone.  Same with Synthroid, synthetic T4, it has to be converted into T3 to be active.




Still with me?




TSH --> T4 from thyroid --> T3 at tissues --> thyroid hormone activity!




Thyroid Regulation




In conventional medicine, a patient is diagnosed as “hypothyroid” when TSH (the brain signal) gets too high.  This is akin to the brain needing to shout at the thyroid to request more hormone, because the thyroid is being sluggish.   However, even if the TSH is “normal” (normal being a relative term – naturopathic doctors have our own ranges for what we like to see for TSH), many patients experience low thyroid symptoms.




Here’s why.




We have to understand that the conversion of T4 to T3 is a very important process, under control of enzymes called deiodinases that remove an iodine molecule from T4 to convert it into T3.  Deiodinase enzymes determine the intracellular activation and deactivation of thyroid hormones.  There are actually three types of deiodinase enzymes – type I deiodinase (D1) and type II deiodinase (D2) increase cellular thyroid hormone activity by converting inactive T4 into active T3.  Type III deiodinase (D3) reduces cellular thyroid hormone activity by converting T4 into the anti-thyroid reverse T3 (rT3).  rT3 binds to T3 receptors but doesn’t turn them on – which is especially bad news, because it takes up space on receptors where active T3 could bind and have an action on cellular metabolism.




D1, D2 and D3 are found at different amounts in different tissues.  Further, the activity of deiodinase enzymes is determined by different physiological conditions at the cellular level.  This means that in different areas of your body, conversion of inactive T4 to active T3 will occur at different rates.




This is a really important point:




Conversion of inactive thyroid hormone  (T4) to active thyroid hormone (T3) happens at different rates in different parts of the body, based on local physiological conditions and the amount of D1, D2 or D3 in that tissue. 




Deiodinase Enzymes




Check this out:


D1 converts T4 to T3 throughout the body, in most of the peripheral tissues (outside of the brain).  D1 is suppressed in response to physiological emotional stress, depression, dieting, weight gain, insulin resistance and diabetes, inflammation, chronic pain, and exposure to toxins and plastics.  In the presence of the above physiological conditions, D1 is less active, and we get a slower conversion of T4 to T3 at the peripheral tissues.  In addition, D1 activity is lower in females, making women more prone to “tissue hypothyroidism.”




D2 converts T4 to T3, and is found in highest concentrations in the pituitary (the same gland that produces TSH).  D2 activity is NOT down-regulated by emotional or physiological stress; in fact, its activity is increased in those above physiological stress conditions (ie. you get better conversion of T4 to T3 in the pituitary when the body experiences stress).  Here’s where the problem with looking just at the TSH to determine if a patient has a thyroid issue is a problem.  In the pituitary, levels of T3 determine the amount of TSH that is released.  If pituitary T3 levels correlated well with tissue T3 levels, this would be a great system.  Unfortunately, they don’t for the above reasons.  So, the brain thinks there’s lots of active T3 floating around, even though the tissues are hurting for T3.




D3 is found throughout the body, but not in the pituitary – which means that reverse T3 (that nasty hormone that pretends it’s T3 but actually has no hormonal activity) only binds and blocks receptors at the tissues.  This further contributes to the problem – TSH looks great! But the tissues suffer for T3.




What To Do?




If you suspect that your tissues are hurtin’ for T3 activity, but your TSH looks normal, you have got to find yourself someone that will look closer at your thyroid.  A naturopathic doctor will run a full thyroid panel, looking at circulating levels of TSH, free T4, free T3, thyroid antibodies, and potentially even a reverse T3 (which we can’t get through Lifelabs in BC). 




Call Acubalance at 604.678.8600 to chat with me on a free 15 minute consult, to discuss your thyroid or other aspects of your fertility.








In health,






Dr. Kali MacIsaac HBSc, ND


Naturopathic Doctor