Thyroid Health: How Your Thyroid Hormones Control Energy, Metabolism, and Mood

The Basics


What it is	A guide to thyroid hormones (T3, T4, TSH) — their roles in metabolism, energy, weight, and mood — and how to evaluate and optimize thyroid function
Primary use	Understanding thyroid health, interpreting lab values, and optimizing thyroid function for energy, metabolism, and overall wellbeing
Evidence level	Strong — thyroid endocrinology is among the most well-studied areas of medicine
Safety profile	Caution Advised — thyroid hormones require careful medical management; self-supplementation carries real risks
Best for	Anyone experiencing unexplained fatigue, weight gain, cold intolerance, brain fog, or suboptimal lab values who wants to understand their thyroid health

⚡ Key Facts at a Glance

The thyroid produces T4 (inactive prohormone) which is converted to T3 (active hormone) primarily in peripheral tissues
TSH (thyroid stimulating hormone) is the master regulator — high TSH indicates the body needs more thyroid hormone (hypothyroid); low TSH indicates excess
Standard TSH testing alone misses many cases of suboptimal thyroid function — full panel (TSH, free T3, free T4, rT3, antibodies) provides better picture
Iodine and selenium are rate-limiting nutrients for thyroid hormone synthesis and T4→T3 conversion respectively
Subclinical hypothyroidism (TSH 2.5-10 mIU/L) affects millions and may cause symptoms even without overt disease

There's a small butterfly-shaped gland sitting at the base of your neck that most people never think about — until everything goes wrong. Your thyroid is arguably the most important regulator of human metabolism, influencing everything from how fast you burn calories to how sharp your thinking is, how well you sleep, and whether you feel like a functioning human being or a soggy loaf of bread.

The Thyroid's Job

The thyroid gland produces two key hormones: T4 (thyroxine) and T3 (triiodothyronine). T4 is the storage form — relatively inactive on its own. It circulates in your bloodstream and gets converted into the active T3 in peripheral tissues like the liver, kidneys, and brain. T3 is the one that actually does the work, binding to receptors in virtually every cell and ramping up metabolic rate, protein synthesis, and mitochondrial function.

This conversion step — T4 to T3 — is where a lot of people silently struggle, even when their basic labs come back "normal."

The TSH Signal

Your pituitary gland monitors thyroid hormones and responds by releasing TSH (thyroid stimulating hormone). Think of TSH as the brain's way of yelling at the thyroid: more TSH means the brain thinks you need more thyroid hormone; low TSH means things are running hot.

The "normal" TSH range according to standard labs is 0.5–4.5 mIU/L. But here's where it gets interesting: many functional medicine practitioners consider anything above 2.0 mIU/L to be suboptimal. A TSH of 4.0 might be "within range" while still leaving you exhausted, cold, and foggy. Optimal isn't the same as normal.

Hypothyroidism: When Things Slow Down

An underactive thyroid (hypothyroidism) is far more common than most people realize. Symptoms include:

Persistent fatigue — even after a full night's sleep
Weight gain — especially around the middle, despite no change in diet
Cold intolerance — always the person reaching for a sweater
Brain fog — poor memory, slow thinking, difficulty concentrating
Hair loss and dry skin
Constipation and slow digestion
Depression and flat mood

The most common cause in developed countries is Hashimoto's thyroiditis, an autoimmune condition where the immune system attacks thyroid tissue. Many people with Hashimoto's go years without a diagnosis because doctors don't test for antibodies unless specifically requested.

Hyperthyroidism: When Things Speed Up

An overactive thyroid (hyperthyroidism) runs the system too hot. Symptoms include anxiety, rapid heartbeat, unexplained weight loss, heat intolerance, and insomnia. Graves' disease — another autoimmune condition — is the leading cause, and it can be life-threatening if untreated.

The Conversion Problem Nobody Talks About

Here's what most standard lab panels miss: you can have a normal TSH and normal T4, yet still feel terrible because your body isn't converting T4 into active T3 efficiently.

Chronic stress is a major culprit — elevated cortisol actively blocks T4→T3 conversion and promotes the production of Reverse T3 (rT3), an inactive form that competes with real T3 for receptor binding. It's like a key that fits the lock but doesn't turn it. The result is functional hypothyroidism even when the numbers look fine on paper.

What Your Thyroid Needs

Several nutrients are essential for thyroid hormone production and conversion:

Iodine — the raw material for T3 and T4. Found in seafood, seaweed, and dairy. Deficiency is rare in developed countries but can occur in those avoiding these foods.
Selenium — critical for the enzymes that convert T4 to T3. Just 1–2 Brazil nuts per day provides your daily requirement.
Zinc — supports thyroid hormone synthesis and TSH signaling.
Iron — iron deficiency impairs thyroid peroxidase, the enzyme that produces thyroid hormones.
Vitamin D — low D is associated with Hashimoto's and autoimmune thyroid conditions.

What Hurts Your Thyroid

Chronic stress — cortisol suppresses the entire system
Crash dieting and chronic undereating — the body interprets calorie restriction as famine and downregulates thyroid output to conserve energy
Excessive raw cruciferous vegetables — raw broccoli, kale, and Brussels sprouts contain goitrogens that can interfere with iodine uptake in large amounts (cooking deactivates most of this)
Fluoride — some research suggests it competes with iodine
Environmental toxins — PCBs, BPA, and certain pesticides act as endocrine disruptors

The Bioenergetic Lens

From a bioenergetic or Ray Peat perspective, thyroid function is the master regulator of oxidative metabolism. A well-functioning thyroid means efficient mitochondria, good CO2 production, warm extremities, and a strong resting pulse. Cold hands and feet, low morning body temperature (below 97.8°F), and a resting heart rate under 60 bpm are all potential signs of sluggish thyroid function — regardless of what a lab panel says.

Labs Worth Requesting

Don't settle for TSH alone. Ask your doctor for:

TSH (ideally 1–2 mIU/L)
Free T3 (the active hormone)
Free T4 (the storage hormone)
Reverse T3 (the blocker)
TPO antibodies (Hashimoto's marker)
TgAb (thyroglobulin antibodies — another Hashimoto's marker)

Lifestyle Foundations

Before chasing supplements or medication, get the basics right:

Eat enough — chronic calorie restriction suppresses thyroid output. Don't crash diet.
Sleep — thyroid hormone secretion follows a circadian rhythm; poor sleep disrupts it
Manage stress — not optional if you want conversion to work properly
Exercise, but don't overtrain — excessive exercise without adequate recovery can suppress thyroid function

Your thyroid is a mirror of your overall metabolic health. Take care of the foundations, test comprehensively, and don't let a "normal" lab result override how you actually feel.

What the Experts Say

Opinions below are paraphrased from each expert's public work, interviews, and podcasts — not direct quotes.

🧠 Andrew Huberman

Andrew Huberman has covered thyroid function in the context of metabolism and energy regulation, discussing how thyroid hormones regulate cellular metabolism broadly. He's noted the inadequacy of TSH-only testing and the importance of free T3 and T4 levels for a complete picture of thyroid health.

⚡ Dave Asprey

Dave Asprey has discussed thyroid health as a critical performance variable, noting that suboptimal thyroid function is extremely common but frequently missed by standard TSH testing. He recommends comprehensive thyroid panels (free T3, free T4, reverse T3, antibodies) and has discussed the importance of selenium, iodine, and avoiding goitrogens for optimal thyroid function.

🔬 Dr. Raymond Peat

Dr. Raymond Peat is one of the most widely read voices on thyroid optimization, and it represents perhaps his most central area of focus. He argues that adequate thyroid function is the foundation of metabolic health, that TSH alone is an insufficient diagnostic measure, and that many people deemed "normal" are functionally hypothyroid. He views the widespread use of T4-only treatment (Synthroid) as inadequate, preferring combined T3/T4 approaches, and has written extensively about the connection between thyroid function and virtually every chronic disease.

Sources & Further Reading

Garber JR, et al. "Clinical practice guidelines for hypothyroidism in adults." Endocrine Practice. 2012. https://pubmed.ncbi.nlm.nih.gov/23246686/
Bianco AC, et al. "Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases." Endocrine Reviews. 2002. https://pubmed.ncbi.nlm.nih.gov/12414823/
NIH. "Thyroid Disease." National Institute of Diabetes and Digestive and Kidney Diseases. https://www.niddk.nih.gov/health-information/endocrine-diseases/hypothyroidism

Where to Buy / Find This

Thorne Iodine & Tyrosine — Supports thyroid hormone synthesis; use only if deficient — https://www.thorne.com/products/dp/iodine-tyrosine