Traditional thyroid testing is limited to TSH. TSH is the hormone produced in your brain that tells your thyroid to produce actual thyroid hormone, which is T4. T4 is still only about 20% of the active thyroid hormone, though; the other 80% is T3. Most of the T4 produced by the thyroid gets converted to T3 in peripheral tissues, especially in the liver and in the gut.

Hypothyroidism diagnoses traditionally depend upon the TSH. There is a feedback mechanism between T4 and TSH: if T4 is low, the pituitary shouts at it louder (and thus, TSH goes up). If T4 levels are adequate, this should not happen.

But what if the problem is really further downstream? Since 80% of thyroid function is dependent upon T3, poor conversion from T4 to T3 can result in hypothyroid symptoms as surely as poor production of T4 in the first place.

I typically test free levels of thyroid hormones, because these are the only ones that are available to stimulate the receptors. So if your fT3 is too low (and my cutoff for optimal functioning is 3.0 ng/dL), why might that be? Here are couple of possibilities.

Your Liver Is Backed Up

Since the majority of T4 to T3 conversion occurs in the liver, logically, that would be the first place to look for a problem. I wrote here on the complex interconnection between the thyroid and the liver. If the liver is backed up (and you might be able to tell if that’s the case with these signs), this is likely the problem. Some liver cleansing is in order.

Your Microbiome Needs Some Work

While dysbiosis (overgrowth of bad gut flora) as a cause of poor T4 to T3 conversion is just now being investigated in the literature, they do acknowledge that “Functional thyroid disorders were associated with bacterial overgrowth and a different microbial composition.” For this reason, SIBO, yeast overgrowth, or dysbiosis generally (bad bacteria in the gut) may be at the root of the problem.

You’ve Got Mineral Deficiencies

Probably the most critical nutrient for conversion of T4 to T3 is selenium. The second most critical would be iron (I wrote here on the interconnected relationship between thyroid and iron levels.) Zinc is also an important mineral for T4 to T3 conversion.

You’re Stressed Out

Stress raises cortisol levels, and cortisol will encourage T4 to turn into reverse T3, rather than T3. High reverse T3 puts the breaks on metabolism just as surely as does low T3.

You’re On a Low Carb Diet

For many people, low carb diets are great for weight loss. For many more, they’re great for awhile, and then they stop working. This is because the body’s glucose needs may exceed intake, leading to a cortisol spike (see above). I wrote more about this process here.

You’ve Got Inflammation Somewhere

The whole point of reverse T3 is to slow down metabolism and allow the body to heal in times of high stress or illness. Cytokines, the body’s chemical messengers for inflammation, therefore will not only spike cortisol (there it is again), but will also inhibit conversion of T4 to T3. The presence of inflammation indicates a problem that needs to be found. If none of the other possible causes sound like you, testing for inflammatory markers might be in order.