Thyroid School, Issue #10// The T4 to T3 Conversion

Welcome back!

I wanted to spend time in this Thyroid School newsletter discussing one of the most crucial, yet most skipped over, aspects of thyroid health… the T4 to T3 conversion.

Let’s recap a bit, shall we?

The thyroid gland produces a few different hormones. The most important ones to recognize for the purposes intended here are T4 and T3.

T4 hormone is produced in higher amounts. Some studies show that T4 makes up about 70-80% of actual thyroid hormones circulating around. Despite it being produced in higher amounts, it’s relatively inactive.

T3 hormone, on the other hand, is produced to a lesser extent (about 20% per some studies), but is way more active in the body. T3 is what binds to cell receptors (which, BTW, all cells in the human body have thyroid hormone receptors!).

So, why would something so important be produced less? Trust me, it’s by design!

T4 converts into T3, but this conversion is delicate and temperamental.

Here are some of the biggest factors that affect this T4 to T3 conversion.

1. Stress. When faced with chronic stress, the body produces Reverse T3 which basically opposes T3. This is a built-in protective mechanism to slow down metabolism when our still primitive body perceives stress. This sends your body into fight/flight/freeze mode instead of rest/digest/reproduce/metabolize.

2. Nutrient deficiencies. Converting T4 to T3 requires selenium, zinc, copper, and magnesium… plus a few other vitamins and minerals along the way. Additionally, adequate protein and carbohydrate consumption is essential for this to happen seamlessly. This is a big reason why extreme diets that omit or decrease macronutrient groups are not recommended for hypothyroidism. 

3. Poor gut function. A lot of the T4 to T3 conversion takes place in the liver and intestines. Poor bacterial balance, gastrointestinal inflammation, or poor liver function can impair this conversion.

So what can you do about this?

1. Test your thyroid numbers. Getting a full thyroid panel that includes total T4 and T3 and FREE T4 and T3. The “free” form will reflect unbound, free to utilize thyroid hormone. It’s helpful to have both values.

2. Assess your micronutrient status. This can be done via blood test that I run on Every.Single.Client because it is THAT important!

Why should you care?

The most commonly prescribed thyroid replacement medication (Synthroid, Levothyroxine) is in the T4 form. If you’ve been getting your doses increased consistently over time, you may want to consider WHY you’re needing more.

1. Is it overall poor T4 production?

2. Is it because of poor T4 to T3 conversion?

3. Are your cells resistant to the thyroid hormones (for example, if reverse T3 is high and you’re not utilizing it well? Or are there nutritional deficiencies that are gunking up the hormone + cell receptor link up?)

Don’t settle for brain fog, fatigue, and all the other yucky things hypothyroidism can lead to. 

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