Low Free T3: 5 Reasons Your Body Isn’t Producing Enough Active Thyroid Hormone

Low free T3 thyroid hormone levels can lead to fatigue, brain fog, difficulty losing weight, constipation, hair loss, and even irregular menstrual cycles. If you’re experiencing these symptoms but your doctor tells you your thyroid function looks “normal,” you’re not alone. 

Here’s the problem: standard thyroid testing often overlooks free T3, the active form of thyroid hormone. Without enough free T3, your metabolism, digestion, mood, and even your brain all slow down. Let’s break down the different thyroid hormones and the five biggest reasons your body may not be making enough active thyroid hormone.

Thyroid Hormones Explained

Your thyroid gland is often referred to as your body’s thermostat, in charge of regulating metabolism, energy production, and so much more. But it’s not just about one hormone, there’s a whole system at play!

Thyroid-stimulating hormone (TSH) is produced in the pituitary gland and signals the thyroid to make thyroid hormones. While TSH is not a thyroid hormone and doesn’t directly indicate thyroid activity, it is often used as a marker to assess thyroid function. While standard lab ranges consider TSH to be normal up to 4.5 mIU/L, within functional medicine we consider the optimal range to be 0.5-2.5 mIU/L. Higher TSH levels correlate to slower thyroid function.

T4 (thyroxine) is the main hormone produced by the thyroid, but it is inactive until converted to T3 (triiodothyronine) in other parts of the body. T3 therefore is our active thyroid hormone, which drives metabolism, digestion, fertility, and menstrual cycles. The best way to measure these hormones is by testing Free T4 and Free T3 (as opposed to Total T4 or T3). Free thyroid hormone levels indicate the amount of hormones that are available for use. Ideally, Free T4 is above 1.0 ng/dL and Free T3 above 3.0 pg/mL.

As you can see, thyroid activity is not limited to the activity of your thyroid gland itself, but it’s largely dependent on how well your body is able to activate thyroid hormones. Let’s dive into 5 reasons you may not have enough active thyroid hormone (Free T3) – one of these might just be the reason you’re experiencing hypothyroid symptoms despite your TSH levels being normal.

5 Reasons You May Have Low Free T3 – Active Thyroid Hormone

1. There Isn’t Enough Free T4 to Make Adequate Free T3

Free T4 is the raw material needed to produce Free T3. Without enough resources, the body cannot keep up with sufficient Free T3 levels. Thyroid function slows down and T4 levels decrease often because of nutrient deficiencies, chronic energy or carbohydrate restriction, Hashimoto’s thyroiditis (an autoimmune thyroid disease), and pituitary issues that impact TSH signaling. 

This is where comprehensive thyroid testing becomes very valuable, so we can see all pieces of the thyroid puzzle. In doing so, we can determine if this is a brain signaling issue, a thyroid function issue, or a downstream thyroid hormone activation issue.

2. Reverse T3 Is Being Produced Instead

When your body is stressed or sick, it activates a protective mechanism, and instead of making Free T3, it converts Free T4 into Reverse T3. Reverse T3 is the deactivated form of thyroid hormone. It has the opposite effect of T3 and actually blocks T3 receptors, slamming the brakes on metabolism in order to conserve energy.

Common triggers that activate this shift from Free T3 to Reverse T3 include chronic stress and chronically high cortisol levels, severe dieting and carb restriction (another form of stress on your body), and inflammation or acute illness. It’s worth noting that you want to avoid testing these levels while you are sick or injured, as we would expect these to increase Reverse T3.

3. Missing Nutrients for Conversion

Your body relies on a handful of nutrients – selenium, zinc, iron, magnesium, and iodine – for optimal thyroid function and the conversion of T4 to active T3. Selenium, zinc, and magnesium all promote thyroid hormone conversion or activation. Low iron (or ferritin, which is the stored form of iron) can increase TSH levels, correlating to slower thyroid function and therefore low thyroid hormone production. 

Iodine is an essential component of thyroid hormones, but too much or too little can disrupt thyroid function. It’s important to know your levels before supplementing, to ensure we are supporting balanced iodine as opposed to potentially driving oxidative stress and inflammation. Without these nutrients, your thyroid lacks the building blocks it needs for optimal function.

4. Impaired Liver or Gut Function

About 60% of thyroid hormone conversion and Free T3 production happens in the liver, and another 20% in your gut with the help of beneficial gut bacteria. If there seems to be a bottleneck preventing Free T4 to be converted to Free T3, we need to assess factors impacting liver function, bile flow, and imbalances within the gut microbiome. 

Your liver is also responsible for detoxifying all waste products for your body to eliminate. If your liver is under an increased toxic burden from issues like mold illness, heavy metal toxicity, fatty liver disease, or high amounts of endocrine disrupting chemicals, thyroid hormone conversion in the liver slows down.

5. High Cortisol is Taking Center Stage

Chronic stress activates the HPA (hypothalamic-pituitary-adrenal) axis and raises cortisol, which tells the body to conserve energy by slowing metabolism. One way it does this is by reducing T4-to-T3 conversion, leaving you with less active thyroid hormone. 

Over time, this stress response can mimic or worsen hypothyroid symptoms. Signs that your low thyroid hormone issue may actually be rooted in HPA axis dysfunction include chronic fatigue, feeling “tired but wired” at night, and caffeine dependence. 

Support a healthy stress response through stress management strategies, stabilizing blood sugar, and prioritizing quality sleep. Adequate carbohydrate intake is also key, as carbs are required for T3 production and healthy cortisol levels. 

Ready To Get a Comprehensive Look Into Your Thyroid Function?

As you can see, thyroid hormone activity goes far beyond your thyroid function itself. If your TSH is normal but you’re still struggling with fatigue, weight loss resistance, mood swings, poor digestion, or irregular cycles, it’s time to look deeper. Your symptoms matter, and you don’t have to settle for feeling tired and foggy all day. 

Standard labs often miss key pieces of the thyroid puzzle. Explore our comprehensive Thyroid Testing Package to take the guesswork out of your thyroid function and thyroid hormones. If you know your thyroid hormone levels are low, consider 1:1 Coaching to uncover what’s driving your symptoms and for personalized guidance and support.

Written by Romana Brennan, MS, RDN