I frequently hear people say things like, “I have a fast metabolism”, or “I don’t know what’s going on with my weight gain, my metabolism is slow I guess”. Well you don’t have to guess. There are clues that can easily be obtained on a thyroid panel. The only problem is, typical bloodwork only tests for TSH. While that is an important marker, it doesn’t tell the whole picture. 

TSH (thyroid stimulating hormone) comes from the pituitary gland in the brain and tells the thyroid to make the raw thyroid hormone, free T4 (thyroxine). It’s an inverse marker meaning that if it’s high, your thyroid is slow (hypothyroidism). If it’s low, your thyroid is fast (hyperthyroidism). TSH is a negative feedback loop from the thyroid to the brain. So it represents the thyroid’s need for stimulation. 

Once raw T4 is made by the thyroid, it’s converted to T3 (triiodothyronine). T3 is the hormone that enters cells and signals them to produce energy, regulate body temperature, and support processes such as heart rate, digestion, and fat oxidation. Because of this, metabolism depends less on how much T4 you have and more on how effectively your body turns T4 into T3.

Since most T4 to T3 conversion occurs in the liver and other peripheral tissues, this can reflect broader metabolic conditions rather than a primary thyroid problem.

Conversion efficiency is influenced by several physiological factors. Adequate calorie intake, sufficient protein, and key micronutrients such as selenium, zinc, and iron all support the enzymes responsible for conversion. Chronic stress, inflammation, illness, and very low calorie dieting can suppress this process, shifting the body toward conserving energy rather than burning it. In these cases, bloodwork may reveal normal or even high T4 alongside suboptimal T3, aligning with symptoms like fatigue, cold sensitivity, or stalled weight loss.

Another useful clue comes from the ratio between free T3 and free T4. While lab reference ranges vary, a relatively low T3 compared to T4 can hint that metabolism is being downregulated. This does not automatically indicate disease, but it can signal that the body is under strain or prioritizing survival over performance. For people focused on metabolic health or athletic performance, this ratio can help contextualize symptoms that might otherwise be dismissed.

Using T4 to T3 conversion as a lens encourages a more dynamic view of thyroid function. Instead of asking whether a single hormone is normal, you begin asking whether your body is using thyroid hormone effectively. Bloodwork becomes less about passing or failing a test and more about understanding how your body is adapting to its current environment.

How to improve T4 to T3 conversion:

Improving T4 to T3 conversion is less about forcing your thyroid to work harder and more about creating conditions where your body feels safe enough to use energy. Since this conversion happens mostly in tissues like the liver, gut, and muscles, lifestyle factors often matter just as much as thyroid specific treatment.

One of the biggest levers is eating enough overall. Chronic calorie restriction, long fasts, or aggressive dieting can signal the body to slow metabolism, even if weight loss is the goal. When energy intake is too low for too long, the body tends to convert less T4 into T3 as a protective response. Consistent meals with adequate carbohydrates and protein can help reverse that signal over time.

Protein intake is especially important. The enzymes that convert T4 into T3 are protein based, and low protein diets can impair their activity. Aiming for regular protein at meals supports not only conversion but also blood sugar stability, which indirectly reduces stress hormones that interfere with thyroid function.

Micronutrients also play a role. Selenium is required for the deiodinase enzymes that convert T4 into T3, and zinc and iron support thyroid hormone transport and metabolism. Deficiencies do not always show up as extreme lab abnormalities, but even mild shortfalls can reduce efficiency. Getting these nutrients from a balanced diet is ideal, but sometimes supplementation may be helpful. 

Stress management is another major factor. High and persistent stress raises cortisol, which can block T4 to T3 conversion and shift more hormones toward inactive forms. Improving sleep quality, reducing overtraining, and building in recovery time can have a positive impact on thyroid labs. This is especially relevant for people who exercise intensely while undereating or under sleeping.

Gut and liver health matter more than most people realize. Since much of T4 to T3 conversion occurs in the liver, factors like alcohol intake, chronic inflammation, or poor blood sugar control can interfere. Supporting liver function through adequate nutrition, minimizing excessive alcohol, and addressing insulin resistance can improve conversion indirectly.

T4 to T3 conversion does not usually improve overnight. It responds to consistent signals that the body has enough fuel, nutrients, and recovery. When those signals are present, bloodwork often begins to reflect a metabolism that is more willing to produce and use energy.

Thyroid Supportive Foods: 

Selenium rich foods - Selenium is directly required for the enzymes that convert T4 into T3.

• Brazil nuts (very high, one or two per day is plenty)
  
  
• Sardines
  
  
• Salmon
  
  
• Tuna
  
  
• Shrimp
  
  
• Eggs
  
  
• Beef, especially grass fed
  
  
• Turkey and chicken
  
  

Iodine rich foods - Iodine is essential for making thyroid hormone in the first place. Too little can impair output, while too much can be problematic, so balance matters.

• Seaweed like nori, wakame, and dulse
  
  
• Iodized salt
  
  
• Cod
  
  
• Shrimp
  
  
• Dairy products such as milk, yogurt, and cheese
  
  
• Eggs
  
  

Zinc supporting foods - Zinc helps with thyroid hormone transport and receptor activity.

• Oysters
  
  
• Beef
  
  
• Lamb
  
  
• Pumpkin seeds
  
  
• Chickpeas
  
  
• Cashews
  
  

Iron supporting foods - Iron deficiency can reduce T4 to T3 conversion even if thyroid labs look normal.

• Red meat
  
  
• Liver
  
  
• Sardines
  
  
• Spinach (especially when paired with vitamin C)
  
  
• Lentils
  
  
• Beans
  
  

Foods that support liver function and overall conversion - Since much of T4 to T3 conversion happens outside the thyroid, these help indirectly.

• Fruit, especially citrus and berries
  
  
• Root vegetables like potatoes and sweet potatoes
  
  
• White rice and other easy to digest carbs
  
  
• Olive oil
  
  
• Avocados
  
  

One way to think about this is that conversion thrives in a well fed, low stress environment. Regular meals with enough protein, carbohydrates, and micronutrients tend to support thyroid hormone use far better than extreme diets or food avoidance. If you already eat many of these foods and still struggle with conversion, that can be a useful signal to look at stress, sleep, training load, or underlying deficiencies rather than just adding more supplements.

Check out our comprehensive panel:

https://www.dcfmed.com/Comprehensive-Blood-Panel