How to Optimize Thyroid Hormone Conversion

This post is adapted from a chapter in my best-selling Essential Thyroid Cookbook: Over 100 Nourishing Recipes for Thriving With Hypothyroidism and Hashimoto’s. Enter your name and email to the left (scroll down a bit) for a sample cookbook.

[Minor updates: March, 2022 and November, 2022; major updates in May, 2024]

Often, when we think about optimizing thyroid function, we tend to focus on getting thyroid hormones balanced, whether that’s with thyroid hormone replacement or nutritional/lifestyle modifications—or both.

While optimizing thyroid hormones is obviously necessary, one missing piece of the conversation is often thyroid hormone conversion, which doesn’t happen for each person in the same way. A stunted conversion process can result in misguided treatment and ongoing hypothyroid symptoms, even in the face of best efforts.

T4 is mostly inactive (“the lame duck”) and is the forerunner to T3, the predominant and more active hormone (“the big daddy”). If the body is to utilize thyroid hormones properly, the wheels that grease the T4 to T3 conversion need to be well oiled. 

Imbalances in any of the areas below could be why you’re not benefitting from your T4 medication. I hear it all the time…I’m on Synthroid, but I don’t feel any differently. My labs are the same (or worse) and my doctor just keeps increasing my dose.

Synthroid is just synthetic T4 (monotherapy), which is what makes conversion especially important for people on T4-only thyroid hormone replacement. (Synthroid is a brand name of Levothyroxine and other brand names include: Tirosint, Levoxyl, Unithroid, and Levo-T.)

How conversion gets compromised

These can also be triggers for Hashimoto’s, so you could consider these factors therapeutic as well as preventative!

- Gut dysbiosis: An Elimination Provocation experiment will largely address this, although I highly suggest a fecal analysis (which I offer as part of my private coaching program) to determine if any gut infections are present and to look at other markers, such as zonulin, calprotectin, anti-gliadin IgA, and SIgA, to name a few.

- Mineral deficiencies, especially zincselenium, iron *, and iodine **: My cookbook goes into detail about how these micronutrients benefit the thyroid and immune system. Go here for the intro to my full Minerals and Your Thyroid blog series.

* Ferritin is our iron storage protein and it’s often low in the Hashimoto’s/hypothyroid community. This is the reason that so many functional/integrative medicine doctors test for it in a full thyroid panel. If it’s low, it can also compromise conversion. So you always want to test for both iron and ferritin.

** Iodine is a bit of a wildcard, insofar as supplementation. We definitely need it for thyroid and breast health, but overdoing it can result in compromised conversion and other problems, which I address in part two of my two-part Iodine series.

- Exposure to toxic halogens like fluoridebromine, and chlorine: I recently updated this oh-so-important post: The Many Hidden Dangers of Fluoride.

- Inadequate beneficial gut bacteria: A whopping 20 percent of T4/T3 conversion occurs in the presence of friendly gut bacteria. I highly recommend probiotic supplementation, with the consideration that for those with allergy/histamine/mast cell issues, you want to be careful and specific about your probiotic strains, as some are histamine inducers, especially Lactobacillus bulgaricis, Lactobacillus casei, Lactobacillus delbrueckii, and S. thermophilus. I can’t claim that this is an exhaustive list. Go here for a guide.

- Impaired liver function: Another whopping 20 percent of conversion takes place in the liver. With the 20 percent mentioned above from beneficial gut bacteria to the 20 percent you get from optimized liver health, that’s almost half the battle right there. A diet high in fiber, drinking warm lemon water (if tolerable—can incite a histamine response), and the herbs dandelion root and milk thistle can go a long way here. But be sure to read this.

- The presence of yeast/candida

- Radiation exposure

- Blood sugar imbalances, including diabetes

- Chemotherapy

- Alcohol consumption

- Estrogen dominance

- The use of birth control pills

- Systemic inflammation

- Low tyrosine, often due to inadequate protein intake: My cookbook goes into detail about the importance of tyrosine.

- High cortisol/adrenal dysfunction/HPA (hypothalamic-pituitary-adrenal) axis dysfunction: Go here for my Restore Your Adrenals guide.

- Heavy metals exposure, especially mercury: Heavy metals are also a major trigger for autoimmunity and so many other conditions and imbalances. Go here for the single best way to detox metals from your body.

Is Reverse T3 really necessary?

For years, the debate about RT3 testing has rolled on—as in, how important is this marker? I feel it’s super important, but many doctors refuse to test it. I have access to functional thyroid testing for my clients and RT3 is on nearly every “complete” or “comprehensive” thyroid panel.

I learned in 2008 that it should always be measured for two reasons:

1. RT3 is just that—the “reverse” of T3, the “big daddy” thyroid hormone. High levels can block thyroid receptors and cause people to be unresponsive or resistant to T3.

2. High RT3 is a sign that the adrenals need a fair bit of support. Go here for my ebook, Hypothyroidism and Adrenal Health: The Often Misunderstood Link.

I’ve since learned that, you guess it, high RT3 can block thyroid hormone conversion.

Go here for a functional thyroid panel, with reference ranges >

Addressing the considerations above is just “good medicine” anyway. As I said above, they can all be triggers for Hashimoto’s and I’d say…any autoimmunity. All are a recipe for optimizing your health and living your best life. The added benefit is that you’ll get way more mileage from any efforts—pharmaceutical or nonpharmaceutical—in optimizing your thyroid health. 

If you’d like to talk through any of this, I invite you to schedule a Jumpstart session with me. Rest assured, it IS a coaching session and at the end of the conversation, I’ll share with you what it would look like to move forward with a private coaching program. 


When you mention radiation, I automatically think wifi, EMF, and computer screens. Are they a source of concern?

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