T4 to T3 Conversion: Factors That Decrease Thyroid Function (& How to Prevent it)

T4 to T3 Conversion: Factors That Decrease Thyroid Function

This is lesson #5 in the thyroid beginner series. 

As you know, this series is designed to help you understand the most basic concepts about thyroid function so you can be a better advocate for your own health

Today is all about T4 to T3 conversion. 

This is the process by which your body creates and activates thyroid hormone. 

It's important because there are steps that you can take to actively enhance this conversion process and help reduce (or improve) your symptoms. 

Let's jump in: 


T4 to T3 Conversion Explained in Plain English

This is really a continuation of the concepts that were built upon in lesson #3 and lesson #4 which discuss T4 and T3 thyroid hormones. 

They are short videos (less than 10 minutes each) so if you don't feel like you have an understanding of thyroid hormones then go watch those before you jump into this lesson. 

As I stated previously, T4 to T3 conversion is the process by which your body takes inactive thyroid hormone and activates it. (1)

This process is also known as thyroid conversion or peripheral thyroid conversion but don't let this confuse you because they are all referring to the same process. 

In this conversion process, your body takes T4 thyroid hormone, that it either produces on its own or that you've taken by mouth, and cleaves off a portion of the molecule which changes its shape and structure. 

This change alters the hormone to T3 (triiodothyronine) which is the active thyroid hormone. 

Why is this important?

Because the more T3 you have in your body the better your thyroid functions

The less T3 you have in your body the more symptoms you will experience

What that means for you is that you want this process to be working at 100% because if it isn't then you will be producing less active thyroid hormone. 

It turns out that the way that your body increases or decreases this conversion process is through the activity of special enzymes. 

These enzymes, known as deiodinase, are responsible for "cleaving" off an iodine moiety on the T4 hormone. 

T4 is named T4 because it contains 4 iodine portions on the hormone. 

what is t4 to t3 conversion

Once it loses an iodine portion it now only has 3 and we call it T3. 

Approximately 80% of the T3 in your body is created through this conversion process which means it's very important for thyroid health. 

This conversion process matters if you have a thyroid, if you don't have a thyroid, if you have Hashimoto's, if you have thyroid nodules, if you have thyroid cancer and so on. 


Because even if your thyroid isn't working you still have to take thyroid hormone by mouth and most thyroid medications contain T4 (not T3). 

Download my Free Resources:

Foods to Avoid if you have Thyroid Problems: 

I've found that these 10 foods cause the most problems for thyroid patients. Learn which foods you should absolutely be avoiding if you have thyroid disease of any type. 

How to Calculate "Optimal" Free T4, Free T3, & Reverse T3 Ratio: 

Calculating these ratios is important because it can help you determine if your efforts are on the right track and whether or not your medications are working. 

Download more free resources on this page

Factors that DECREASE T4 to T3 Conversion (These slow down thyroid function)

There are certain factors which can slow down the conversion of T4 to T3 by interfering with the efficiency of the enzymes responsible for this conversion.

These factors act as a brake on your entire thyroid system. 

They slow down the amount of T3 that your body is able to produce and may make less efficient the medication that you are taking.  

If you can address and reverse these issues then you can "release" the brake on your thyroid and finally start feeling better! 

I've created a list of the most common factors which blunt this conversion process below: 

  • Dieting & Calorie restriction (slows down your metabolism) - Calorie restriction, as little as 1,500 calories per day for 3 weeks, is enough to blunt thyroid function and reduce T4 to T3 conversion. (2) This might be the #1 problem for many thyroid patients who also tend to struggle with weight. They get trapped in a situation where they are overweight but try to diet to reduce the weight, only to further reduce thyroid function which leads to weight gain. 
  • Inflammation (from any cause) (3) - Inflammation is really nonspecific but inflammation from any cause (if systemic) can damage thyroid function. You can identify inflammation by looking for markers such as CRP and ESR. If elevated, these may be an early sign of inflammation. 
  • Lack of sleep (4) - If you are sleeping less than 8 hours per night then you may be reducing your thyroid function without realizing it. 
  • Intestinal issues (Dysbiosis, IBS/IBD, etc.) - A large portion of thyroid conversion actually happens in the GI tract or gut. (5) If you have problems in the gut then this may limit your ability to convert T4 to T3. Problems such as intestinal dysbiosis (changes in gut bacteria), inflammatory conditions such as SIBO/SIFO/IBD, and other issues can all cause issues. Treating gut related issues should be a priority if you have thyroid disease. 
  • Stress (emotional, physical, etc.) - Stress, from any cause, will limit thyroid function and block thyroid conversion. (6)
  • Chronic and acute infections - Acute infections such as pneumonia can cause issues but also more chronic infections such as viral or bacterial overgrowth syndromes can also cause issues. 
  • Medications - It is well known that certain prescription medications can block thyroid function. (7) Included on this list are blood pressure medications, diabetic medications, seizure medications, iodine, PTU/Methimazole, steroids, and heart medications like amiodarone. It may not be possible for you to stop taking your medication but you should look for alternatives if possible. 
  • Endocrine disrupting chemicals (those you come into contact with daily, some people more sensitive than others) - Endocrine disrupting chemicals are chemical compounds that humans come into contact with on a daily basis which look like real hormones but aren't. These hormones can trick your body into thinking the real hormones are there and they may compete for binding on cellular receptors. (8) EDC's are found in plastic bottles, on receipts, in foods and so on. Limiting your exposure to these chemicals may help improve your thyroid function. 
inflammation reduces t4 to T3 conversion

If you have any of these issues going on in your body then your thyroid function may suffer. 

These conditions may lead to an increase in a metabolite known as reverse T3 (a topic for tomorrow). 

Reverse T3 is the metabolite created when your body is having trouble with the thyroid conversion process. 

Therefore, it can be used as a marker of thyroid conversion in your body. 

You can test for reverse T3 with a simple blood test. 

Factors that INCREASE T4 to T3 Conversion (You can use these to improve how well your thyroid works)

So what are you supposed to do if you have any of the factors listed above?

This section will discuss some of the available therapies that you can use to potentially help improve T4 to T3 conversion. 

  • Fixing the problems listed above (you must know what is causing the issue!) - The first, and most important, option is to try and find the root cause of your issue and address that problem. This may require a physician to help but in many cases, it will not. For instance: if you know you are struggling with calorie restriction then the first step is to stop that behavior! If you have known gastrointestinal issues then you can address that problem with basic therapies such as probiotics or enzymes. If you are struggling with stress then adding yoga or meditation to your daily regimen can help. If you are having issues with sleep then you can focus on ways to improve that and so on. Some issues, such as the use of medications, may require that you find a physician to help (and even then you may not be able to adjust your medications). In many situations, you may find that you have a number of factors which are limiting your T4 to T3 conversion. Each of these factors may contribute 5-10% so you may have to address many issues to see noticeable improvement. 
  • The use of certain supplements - Another strategy to improve T4 to T3 conversion is to use certain supplements. Supplements such as zinc, selenium (9) and guggul may all help your body naturally improve this process. The use of minerals such as zinc and selenium will only help if you are deficient, but many people around the world most likely are due to poor widespread dietary patterns. I've included the most effective mix of nutrients and vitamins in my T3 conversion booster which has helped many people improve thyroid function. You can learn more about that supplement here
  • Exercise (the right amount for your body) - Exercise is one of the easiest ways to increase both T4 and T3. A moderate amount of exercise can improve thyroid hormone production from your thyroid gland while simultaneously increasing thyroid hormone levels. The only catch here is that you must be exercising enough while avoiding overexercising. Overexercising can actually cause the opposite effect and lead to decreased levels of both T4 and T3. Learn how much is enough in this guide
  • Diet - While your diet doesn't directly influence thyroid conversion it can absolutely indirectly affect thyroid conversion. Consuming unhealthy foods can lead to things like low-grade inflammation and gut problems. Both of these variables can negatively impact thyroid conversion. Learn which foods to avoid and which foods you should be eating to promote healthy thyroid function in this guide
natural thyroid supplements version 2

Using these strategies can help you increase thyroid function naturally and may even help if you are taking medication. 


If you are taking thyroid medication your body still must convert T4 into T3. 

So all of these factors still matter if you are taking thyroid medication or if you don't have a thyroid at all. 

The good news is that almost everyone with thyroid related issues can benefit from this information. 

Bypassing T4 to T3 Conversion with T3 Medication

Another strategy that we will discuss (briefly) is the use of T3 thyroid hormone to bypass T4 to T3 conversion. 

We already discussed the importance of T3 and how it does pretty much all of the heavy lifting when it comes to thyroid function in your body. 

Normally, thyroid medications which contain T4 must be activated to T3 before your body can really use them

But you can skip out on this process by using T3 medications such as liothyronine or Cytomel. 

If you ingest T3 medications then they do NOT require activation in the way that T4 medications do. 

This allows you to bypass the conversion process and to start feeling better almost immediately. 

This doesn't mean that you should skip out on all of the therapies listed above, but instead combine them all together for maximum benefit. 

I find that using medications which contain T4 + T3, using supplements designed to improve thyroid conversion and addressing issues which blunt thyroid function all at the same time cause the best results clinically. 

You can learn more about using T3 medications here


The bottom line?

Addressing this tiny concept known as thyroid conversion may be one of the most important things that you learn about your thyroid. 

Most of the time, patients feel helpless with their thyroid because so much depends on their doctor and what he/she is willing to do. 

But if you understand your thyroid you can take steps to optimize whatever thyroid function your body is capable of producing

Just remember:

This is a process and it takes time!

Don't expect immediate results, but instead give your body weeks to months to improve. 

Now I want to hear from you:

Are you struggling with T4 to T3 conversion? 

Have you tried therapies designed to improve this process?

Have they worked for you? Why or why not?

Have you tried T3 medications?

Leave your comments and questions below! 

References (Click to Expand)

why T4 to T3 conversion is so important
Dr. Westin Childs

Dr. Westin Childs is a Doctor of Osteopathic Medicine. He provides well-researched actionable information about hormone-related disorders and formulates supplements to treat these disorders. He is trained in Internal Medicine, Functional Medicine, and Integrative Medicine. His focus is on managing thyroid disorders, weight loss resistance, and other sex hormone imbalances. You can read more about his own personal journey here.

28 thoughts on “T4 to T3 Conversion: Factors That Decrease Thyroid Function”

  1. Yes! I’m hoping my endocrinologist will add T3 to my treatment. I wish there were doctors like you in Georgia. I am struggling with just T4. I just want to feel normal.

  2. Recent blood test show I am not converting t4 to t3. But gp says I am ok.i am on 100mcg levothyroxine and but feel awful. 15kg weight gain , hair loss etc. Went to private who recommended 50mcg of t3 , 50mcg of t4, ldn and Dr Wilson adrenal blocker ? Have yet to receive the meds. Will this help ? Blood Results THYROID STIMULATING HORMONE* 4.45mIU/L

    FREE THYROXINE 18.8pmol/l

    FREE T3 3.2pmol/l

    IMMUNOLOGY THYROID ANTIBODIES.Thyroglobulin Antibody 66.5IU/mL

    Method used for Anti-Tg: Roche ModularThyroid Peroxidase Antibodies* 526.0 REVERSE T3* 25ng/dL

  3. I’m on 100mcg levothyroxine and 25mcg Levothyronine (2x 12.5mcg/day). All my thyroid hormones seem to be in the happy middle but I have gained 30lbs on top of all the weight I had gained before…. I wonder if I should stop taking levothyroxine and only continue with levothyronine. May ask my doctor to increase dosage of levothyronine to 2x25mcg/day

  4. I have tried Naturethroid (had major digestion issues), Armour and NP (both Armour and NP made me feel grouchy). I was on NP and started breaking out on my face. I had to go back to levothyroxine and doctor wouldn’t prescribe T3 meds. Currently, my Free T3 (3.1) and Free T4 (1.6) has improved but my TSH is crazy low, .05. Also, my Reverse T3 has gone back up and is now 17. Would the T-3 conversion supplement help. And, should I be concern about my low TSH number.

  5. Hello Dr. Childs, I have 45-50 lbs weight gain, thyroid & adrenal symptoms. I think I need more T3 but am afraid to take it without your guidance. I’ve read to take bile, what do you think of that?
    2001 I was 44 yrs, on TriPhasal, had a lot of stress work/ personal life. I fasted losing 40 lbs, went from 155 lbs to 115 lbs. The weight increased without proper nutrition. HFCS in foods and sugar and dairy, ate salads w/HFCS dressing. Mushrooms and tomato nightshades were stopped. In 2007 about 120 lbs, had an ear infection from impacted wax and it went to the other ear. I took many antibiotics Levequin, Avelox, Bactrim. I lived by a woods which had bacteria and fungus. I was in perimenopause and my sex hormones were imbalanced, I think this was causing my hearing loss. I went on William Crook’s candida diet and moved from the wooded area. I took New Chapter All Flora probiotic which they don’t make anymore, it helped. I continued trying to get better and alleviate my chronic fatigue and weight gain. I thought it was allergies.
    2013 I went to a conventional NP who prescribed BHRT, first 80/20 bi-est which caused breakthrough bleed. She then prescribed .375 estidiol/100mg Progesterone & 5 mg Testosterone. She did not test first, prescribed by how one feels . Some things were good, but I began getting chin hair and aggressive so I tested.
    A NP tested me in 2015- She gave me ZRT test kit for saliva cortisol and blood spot hormone and nutrient deficiency tests; I was low in AM, morning, more normal in afternoon and high at night for cortisol curve. I don’t feel awake in early morning or afternoon and any stress makes me worse. She also ordered blood spot tests for sex hormones and these:
    Ferritin 162 (13-300), B12-1034 (243-894), Folate 20 ( 9.3-30.9), RT3 18.5 (18-25), FT3-3.0 (1.8-4.6), FT4- 1.33 (0.9-1.7), TSH 1.650 (0.27-4.2), TPO AB- 8.1 (<35), TG AB- 124 (<115), Vit D 35 (30-100).
    She diagnosed me w/Hashimotos by Thyroflex & prescribed BHRT 0.2 estriol/0.2 estridiol, 100 mg Progesterone and 5 mg Testosterone.
    She prescribed 5 mg DHEA & adrenal supplements. I began to feel better on the adrenal supplements and BHRT but was afraid to take the 60 mgs NP Thyroid and DHEA. I started those after. I started speeding, heart palps, eye changes, and I started and went off of the NP Thyroid in 2 days and experienced intense undulating pain on my scalp. I continued 5 mg DHEA and had severe loss of facial collagen in cheeks, next to nose, which looked like the women who take steroids. Dark circles and wells under my eyes not previously there. I tried botox in 2015 for the eleven creases between the eyes, had swelling reaction and fat pads under my eyes reduced and moved.
    My TSH fluctuated from 0.57 (.40-4.70) on 11/23/2009; 4.27 (.40-4.70 UIU/ML) on 4/11/2013; 2.42 (.40-4.70 UIU/ML) on 01/23/2018. I have high cholesterol. How do I know if I should go on T3 without causing more speeding and collagen loss? Do you think it was going off of the NP Thyroid or the DHEA? DHEA-S –132mcg/dL (<15-200) 2013 DHEA-S 1.5 ng/ML (2-23) in 11/24/2015

    I'm off BHRT now due to hyperplasia and had pain during and after vaginal ultrasound, am afraid to have another one. I was applying the BHRT cream between thighs. I have stomach & thigh fat, butt is flattening out, eyes are dry from meibum thickening/clogged glands; pre diabetes; high cholesterol; hair thinning; nail ridges (only after starting and stopping NP Thyroid in 2 days), fatigue; limited energy, vertigo from ear infection.

    • Hi Kim,

      It seems that you have a lot going on based on this history here. Because of the complexity, there’s really no way for me to give you advice but what I do think you should do is seek out a physician who is knowledgeable and who can help you further. You can use this as a resource to help get you started: https://www.restartmed.com/thyroid-doctor/

  6. I had my thyroid ablated in 2005 (from Grave’s disease).

    Recently, with the help of my Naturopath, I reduced my usual levothyroxine-only therapy from 150mcg to 137mcg to 112mcg and now 100mcg (and switched to Synthroid), while newly adding (and titrating) T3 to a level of 15mcg. (Long-short: I’m currently on 100mcg Synthroid and 15 mcg liothyronine daily.)

    Q1: Is this a proper ratio?

    Q2: Does taking a T3 med cause more of the T4 med to be converted to RT3, since T3 needs are now being met by T3-only medications? Do RT3 levels go up when patients add T3 to their T4 regimen?

    Q3: Is it worthwhile to take 5mcg T3 three time a day (spaced at 4 hours apart)? I’m looking to get rid of the 2pm energy crash, but don’t want the fast/strong heart rate that taking two 5mcg together seems to give me.

    Ps. I’ve only been on T3 for two weeks. We increased 2.5mcg every 3 days until I reached 15mcg. Hoping that wasn’t too fast, after just reading your article that said to increase every couple of weeks.

  7. Hello! I had a total thyroidectomy due to cancer. Would the conversion supplement assist even though im 100% dependant on medication?

    • Hi Corinne,

      The short answer is yes because your body must still convert the T4 that you take (through levothyroxine or Synthroid) into the active T3 in peripheral tissues.

  8. I switched from .88 Synthroid to 60 mg NP Thyroid and felt great. My dr then added an additional .15 mg NP Thyroid. Now i have chronic diarrhea andcant sleep. She insists i am not hyperthyroid so i went tomy primary and she said to take 0 for 2 days and then resume the 60 mg. I still have diahrrhea, ringing in my ears and cant sleep. Do i need to goback to Synthroid?

  9. Hi Dr.
    My Dr. I go to loves your methods and request we read your articles. She has REALLY helped me on this hypo journey where 4 other Drs wouldn’t listen to me.
    She had me using Saxenda for Leptin resistance and it was working however, my insurance doesn’t cover that drug. It’s too expensive to buy out right.
    What do you recommend in its place?
    Thanks so much!

    • Hi Kelli,

      That’s great! Who is your doctor? I’m going to curate a list of recommended doctors I can recommend so it would be great to add her.

      You can use whatever your insurance will cover in the GLP-1 agonist class. If you call you can ask what medications fit that list and they should tell you which (if any) they cover.

  10. Given the drug companies, (first Armour, then Naturthroid, and sadly now NP), can’t seem to stop changing their formulations and then deny they’ve made any changes, what do you suggest for someone like me? I have no thyroid, ANY outside changes seem to throw me way off. Now NP has joined ranks with the other untrustworthy companies.

    Should I go on synthroid and cytomel combo? I’ve tried to stay far away from synthroid because of how horribly I did on it in the past. For the past 15 years I’ve been on NDT until the changes, shortages, etc.

    I’m frustrated and scared. Right now I’ve switched over to Thyrovanz, but am out here on my own and feeling awful. Afraid to tell my doc that I’ve gone off NP and I’m on a “supplement” instead.

    This is really, really messing with my health. Physically and mentally.

  11. For the past 6-7 months, I’ve had T3 (Cytomel) added to my daily NatureThroid regime. It’s been life changing! I feel alive. I’ve not had any heart issues, hahaha! I do split it in half, taking it twice daily, as per my physician. We ran full panels of tests for a long time with my past physician and my newer physician. It seems like I’ve had a conversion issue for years! After a particularly stressful time in my life, dealing with grief, I found that I had to gradually increase NatureThroid by 2 grains. Yeah, I’m on 3.5 grains now. My genetics are such that thyroid issues run in my family rampant, along with living in Michigan, part of the Goiter Belt. My inflammatory markers and autoimmune ones for Hashimoto’s came back normal constantly.

    One thing though is that not every person needs to eat more than 1500 calories. I am 5’2 in height and only weigh 100-105 pounds, am 47, do yoga and walking, so I don’t require that much fuel. I do eat high quality, low fat, whole food plant-based diet, however. When you eat this way, it’s naturally low calorie.

  12. I had a total thyroidectomy. I wanted to use NDT but found it hard to find. I found a company from Thailand to buy it from. The problem is I have no idea what I’m taking. I’ve tried to find a lab to get it analyzed but can’t find one. I have a feeling it might be just t4. I’ve been thinking I should just take Synthroid and use Cytomel. If I’m on 150 mcg of Synthroid how much Cytomel should I take? Can I just take the Cytomel? Thank you

  13. Dr. Westin,

    If my RT3 is consistently high (21-23) on 2 grains of NP Thyroid, do you think it would make sense to add 25mg cytomel and lower the NP to 1 grain to see if that helps? My dr. typically prescribes T3 with Synthroid, but I’d like to try T3 with NDT if at all possible just due to my past with synthetic. Thanks!

  14. Hi dr, thank you for the insite. I talked with my Dr she reluctantly agreed to try the cytomel. So I’m excited to see what a life feels like again. 30 pounds overweight, fatigue , brain fog, on and on… I’ve had a thyroidectomy and now two years of struggle to fix my levels. Yoyo tsh and high t4 low t3 . I’ll let you know! Thanks AGAIN for this site!

  15. Hello Dr. Childs,

    I had a TT at the end of February. Was started on levothyroxine 100 then switched to 112 then back to 100 due to hyperactivity.
    My TSH was 0.01 and my free T3 was high at around 4.6 and free T4 1.4.
    I most recently switched to Amour Thyroid 60 mg because the levothyroxine made me feel sick and itchy. My question is, since my T3 is higher than my T4, does that mean I am a good converter and don’t need to be on NDT? If I am a good converter, can I still stay on NDT or switch to something else?


  16. Dr. Childs,
    I love reading your blogs and listening to your podcasts. I sometimes feel like I know more about my thyroid than my doctor does! I’m 47yo, work out at least 4-5 times per week but can’t seem to lose a bit of weight. I’m currently taking NP thyroid and your T3 Conversion supplement. My levels are: TSH:1.51, FT3: 3.1, FT4: 7.0 and RT3: 13.6. I’ve also recently purchased your Leptin Resistance supplement, as well as Berberine when my most recent Glucose came back at 103.

    Thoughts on switching to a T3 supplement only?


  17. Hi. I’ve been on Armour for 25+ years. Decided to try Brand Synthroid. Labs show RT3 at 22. FT3 at 3.27 which is low for my liking. It raised my suppressed TSH a bit and FT4 is high. Do you suggest I go back to Armour? I did great on it besides suppressing my TSH. Thank you. I’m a long time follower


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