Thyroid - How does my body make more metabolically active T3?

Your body makes T3 in two main ways.

Your thyroid gland produces some T3 directly, but most of what it releases is T4 (a prohormone).

2.Most T3 is made outside the thyroid by converting T4 to T3 inside tissues using enzymes called deiodinases (D1 and D2, not to be confused with Vit D).

The main sites that make circulating T3 (the stuff that shows up in bloodwork)

Liver
Kidneys

A different enzyme pathway (D3) converts T4 into reverse T3 (rT3), which is inactive and competes with T3 signaling.

So if you “feel hypothyroid” with normal labs, it can be a conversion and signaling problem, not just a “thyroid gland problem.”

What increases T3 (and supports more T4 to T3 conversion)?

1. Stop the biggest T3-lowering triggers

These are the most common reasons the body lowers T3 (often as a protective adaptation).

Severe calorie restriction / starvation-like dieting tends to lower T3 and can raise rT3.
Very low carbohydrate intake can lower T3 in humans; studies show T3 drops when carbs are sufficiently low, and rT3 can rise more with very low carb intake.
High stress,
poor sleep,
illness,
inflammation commonly shift conversion away from T3 and toward conservation patterns (including more rT3).

2. Hit the key “conversion nutrients” (food first)

The deiodinase enzymes and thyroid signaling depend heavily on nutrient status.

Selenium supports deiodinase activity involved in T4 to T3 conversion. Food- seafood, eggs; Brazil nuts in small amounts (easy to overdo, sometimes salty, rancid, high fat).
Iron status matters for thyroid physiology and enzyme function; low ferritin can mimic hypothyroid symptoms and impair performance. Food - red meat, lentils/beans; pair plant iron with vitamin C foods.
Zinc supports thyroid hormone metabolism (often low in restricted diets).Food - meat, shellfish, pumpkin seeds.

If you supplement, it’s smartest to do it based on labs (especially iron). Do NOT self medicate.

3. Support the main conversion “hubs”

Peripheral conversion is heavily influenced by liver and overall tissue metabolism (deiodinase activity is a major pathway).

What to do?

Keep digestion moving (constipation is a common “low thyroid” companion).
Get daily walking and consistent meal timing.
Limit alcohol during a “rebuild metabolism” phase.

4. Train in a way that signals safety. This is hugely important.

Strength training 2-4x/week supports muscle, which supports metabolic resilience and reduces the “catabolic” feel many women get when they’re under-fueled.
Avoid living in high-intensity training if you’re already depleted. Recovery helps conversion.

5. A simple “increase T3” weekly checklist

If you want the highest-yield actions, start here -

Eat consistent meals with protein at each meal
Include some carbs most days (fruit, potatoes, rice, oats, beans as tolerated)
Stop aggressive restriction for 2-4 weeks if you’ve been dieting hard
Sleep routine - same bedtime/wake time most days
Strength train 2-3x/week & walk most days
Selenium + zinc foods weekly; iron addressed if low

One important note (IF you’re NOT on meds)

If symptoms persist, it’s worth checking for the “most common mimics” of hypothyroid feelings which are low ferritin/iron deficiency, low vitamin D, B12 issues, under-eating, chronic stress, and autoimmune thyroid antibodies. These can create the experience of low thyroid even when TSH is “normal.”

Quick “T4 user” checklist if using Meds (Synthroid/ levothyroxine/Levoxyl/Unithroid/ Cytomel/dessicated Thyroid)

Same time daily
Empty stomach (30-60 min before food or 3-4 hr after dinner)
Keep calcium/iron/antacids (aluminum)/high fiber 4+ hours away. They compete with the meds
Delay coffee until after the waiting window (30-60 minutes)
Hold off on taking biotin (Vit B7) 2 or more days before labs. Biotin (common in hair/nail/multi/B complex supplements) can cause falsely high T4/T3 and falsely low TSH on many lab assays.
Recheck labs after any routine change (timing, supplements, diet pattern, formulation)