#373 – Thyroid function and hypothyroidism: why current diagnosis and treatment fall short for many, and how new approaches are transforming care | Antonio Bianco, M.D., Ph.D.

Welcome to The Drive. I’m Peter Attia, and this show, my site, and my newsletter are all about turning longevity science into something you can use. Today’s guest is Dr. Antonio Bianco, a leading thyroid researcher and former president of the American Thyroid Association, who’s spent decades studying how deiodinase enzymes shape thyroid hormone action. We’ll unpack thyroid biology, why TSH alone often misleads, who benefits from T3 plus T4, and why many patients feel unwell despite “normal” labs. With that, let’s jump in.

You’re running a med school and a lab—what’s the central question your research tackles, and what drew you to the thyroid?

We’re mapping what thyroid hormone actually does in each tissue and cell, down to how T3 reshapes chromatin to control gene programs, because that’s how it changes organ function and, ultimately, how patients feel. The goal is better care for people who lack adequate thyroid hormone.

Let’s ground listeners in the basics: the thyroid sits over the voice box, makes mostly T4, and the body converts it to active T3 that drives energy, temperature, mood, and more; when production falters, we replace it. Why the body prefers this T4-to-T3 system?

The gland concentrates iodine from the blood to build thyroid hormone and stores a large T4 pool, then releases it steadily. T3 is the active species, but receptors bind T3 tightly and T4 poorly, so the conversion step determines real biologic effect.

Is that T3 preference about molecular fit?

Yes—the receptor pocket favors T3’s shape, so T4 has low affinity unless levels are extremely high.

From an evolutionary standpoint, is secreting long‑lived T4 a way to distribute an inactive prohormone and let tissues dial in T3 locally?

The bigger pressure seems to be iodine scarcity, so the system evolved to conserve iodine by stockpiling T4 and reclaiming iodide after activation. T4’s half‑life is about eight days, T3’s is roughly twelve hours, which lets the body quickly modulate exposure when needs change.

Walk us through reverse T3 and the deiodinases.

Removing an outer‑ring iodine from T4 yields active T3; removing an inner‑ring iodine makes reverse T3, which is essentially inert at the receptor. Deiodinases route T4 toward activation or inactivation, providing a rapid, tissue‑level control dial.

I used to do long fasts; my free T3 fell, reverse T3 shot up, and TSH rose modestly. What’s happening?

The hypothalamus senses low insulin and leptin as a shortage signal and eases off metabolic drive, so conversion shifts away from T3 toward reverse T3 and the liver clears less of it when D1 activity drops with low carbs and insulin. Energy expenditure declines, which is why weight loss often plateaus after the first days.

Is the free T3 to reverse T3 ratio a useful proxy for overall thyroid activation?

It’s a reasonable surrogate for deiodinase activity when interpreted carefully, since it reflects both production and clearance, and we can’t directly measure deiodinases in blood.

Compare D1, D2, and D3 for us.

D2 is the workhorse for activating T4 to T3 and accounts for most peripheral T3 production; D1 contributes and helps clear reverse T3, particularly in liver; D3 is the brake, inactivating T3 to T2 and turning T4 into reverse T3. Reverse T3 is a metabolic dead end, while iodine is recycled.

Before disease states, explain hypothalamus–pituitary–thyroid regulation and why TSH itself isn’t the problem.

TRH from the hypothalamus drives pituitary TSH, which stimulates the gland to make hormone; TSH doesn’t cause symptoms, the hormone excess or deficiency does. Thyroid hormone levels in blood barely budge day to day, so real regulation happens locally via deiodinases, not by big swings in serum.

So tissue T3 can change while blood looks stable?

Exactly; in cold exposure, brown fat D2 can raise local T3 approximately ten times within hours to drive heat production, with no big change in serum. The brain similarly relies heavily on local T3 via D2.

Can the hypothalamus sense circulating hormone directly?

Yes; key hypothalamic regions that control TRH sit outside the blood‑brain barrier and, along with the pituitary, express D2 to convert T4 to T3 for precise feedback control.

All this physiology matters because treatment dogma varies widely; without it, we risk missing what actually helps and what harms.

Agreed—ignoring T3 makes little sense when it’s the active hormone, yet many clinics never measure it, which reflects gaps in physiologic understanding that can affect care.

On labs, what’s the difference between total and free hormones, and which assays can we trust?

Almost all T4 and T3 in blood are protein‑bound and inactive; only the free fraction enters tissues. Free T4 by immunoassay is generally reliable, but T3 and free T3 immunoassays are variable, especially at lower levels, and reverse T3 assays are even less consistent; mass spec would be preferable for T3, though it’s not widely available clinically.

Pregnancy raises binding proteins, so totals climb while free levels stay steady, which is why we prioritize free measurements. Genetics or sex meaningfully change targets?

Genetics explain some variation but rarely change clinical decisions; men tend to show a tighter TSH range, but the differences aren’t usually actionable.

How common is hyperthyroidism compared to hypo?

Hypothyroidism affects millions of adults in the United States, whereas hyperthyroidism is far less common, seen in hundreds of thousands.

Review hyperthyroidism: typical causes, symptoms, labs, and treatment choices.

Graves disease is an antibody‑driven stimulation of the TSH receptor, causing high T4 and T3, palpitations, weight loss, heat intolerance, tremor, and brisk reflexes; labs show suppressed TSH and elevated hormones, and antibody testing confirms. Treatments include anti‑thyroid drugs, surgery, and radioactive iodine, but use of radioiodine has waned due to cancer concerns, while high‑volume thyroid surgeons have made surgery an excellent option; toxic nodules can be managed similarly, with surgery often favored.

Let’s pivot to the more common problem—hypothyroidism—starting with how it’s usually found today and the leading cause.

It’s now mostly picked up on routine TSH screening before symptoms become obvious. The most common cause is autoimmune destruction of the gland, often marked by TPO antibodies, though not all cases are antibody‑positive.

Replacing hormone assumes the immune attack is limited to the thyroid, yet TPO positivity in pregnancy raises miscarriage and prematurity risk, which suggests broader immune activity.

Right, TPO can signal a more global immune milieu, and it often clusters with other autoimmunity; infertility specialists sometimes lower TPO titers with short courses of steroids, and I’ve seen patients conceive afterward, though robust trials are lacking.

You mentioned patients who felt cognitively blunted despite normal TSH and free T4 after thyroidectomy—what did that change for you?

Hearing multiple professionals lose their edge after surgery and standard levothyroxine, with normal labs, pushed me to redirect my research toward why some remain symptomatic and how tissue‑level thyroid signaling might explain it.

Beyond Hashimoto’s, what else falls under thyroiditis, and which antibodies matter?

Subacute inflammatory thyroiditis can be intensely painful and transiently destructive, but Hashimoto’s is the classic chronic form; TPO is the key antibody, with anti‑thyroglobulin less informative.

Do we treat the autoimmunity directly, and what defines hypothyroidism on labs?

Standard care focuses on hormone replacement, though selenium, vitamin D, and other antioxidants may lower TPO titers and prolong residual function in some, which isn’t yet guideline‑driven. Biochemically, a high TSH with a low free T4 clinches primary hypothyroidism, and many use a TSH above roughly ten with a depressed free T4 as a clear threshold.

So TPO positivity with a mildly elevated TSH and normal free T4 isn’t hypothyroidism yet, but it warrants context, follow‑up, and sometimes ultrasound, especially with strong family history.

Exactly; I often repeat labs within a few months to avoid letting a patient drift into symptoms, and I individualize decisions. A notable minority develop high TSH without detectable antibodies, and causes can include prior thyroid injury or other autoimmune processes not captured by standard tests.

Are there active trials trying to calm Hashimoto’s—like steroids during the honeymoon—to preserve function, or are we still focused on replacement alone?

Let’s get into therapy. We have FDA‑approved T4 and T3, but the standard today is still T4 monotherapy; is that where things stand?

Yes. Levothyroxine is the standard for hypothyroidism; T3 was approved early and historically used around thyroid cancer care, but true T3‑only treatment is rare and not recommended.

Part of why T3 alone is tough is the short half‑life and surge effect, while T4’s long half‑life makes once‑daily dosing simple and forgiving.

Outside FDA‑labeled options, desiccated thyroid is popular and polarizing; can you lay out the best case for and against it?

Desiccated thyroid is powdered pig thyroid that delivers both T4 and T3; potency is now standardized, safety looks similar to levothyroxine, and blinded trials suggest patients often prefer combination therapy.

The concern has been T3 spikes and older manufacturing variability, but modern specs tightened that, while levothyroxine remains simple, consistent, and works well for the majority with TSH‑guided dosing.

We should note levothyroxine was effectively grandfathered without hard‑outcome trials, so we don’t know if normalizing TSH normalizes risk.

Retrospective data show higher mortality in treated hypothyroidism versus healthy controls, likely reflecting incomplete tissue euthyroidism and comorbid autoimmunity, not harm from T4 itself.

Mechanistically, when you’re hypothyroid the liver under‑clears LDL, and fixing TSH may not restore hepatic T3, which could explain the frequent need for statins.

Exactly; animal data show persistent hepatic hypothyroidism on T4 alone, and in the clinic we often need statins, signaling residual metabolic risk.

In a large retrospective analysis, combination T4 plus T3 was associated with about a 30 percent lower mortality than T4 alone, though both groups still carried excess risk.

That could reflect more engaged patients or physicians, but your team tried to balance cohorts with propensity matching and prior‑year hospitalization checks.

Right; we controlled for many factors, though mindset is hard to measure, and the signal still favored adding some T3.

Given that gap, it’s surprising we don’t have a prospective trial; hypothyroidism clearly carries meaningful risk.

It does, and clinicians should view hypothyroidism as a cardiometabolic risk factor that deserves tighter follow‑up beyond just hitting a TSH target.

On diagnosis, some schools call nearly everyone hypothyroid; how do you anchor the workup?

Diagnose with TSH and free T4; T3 is usually normal early and not helpful, symptoms are nonspecific, and true secondary hypothyroidism is rare and requires a low free T4 with an inappropriately normal or low TSH.

Lower body temperature can occur in hypothyroidism, but a low temperature alone doesn’t prove it, and menopause commonly mimics thyroid symptoms, so labs have to lead.

What about compounded controlled‑release T3 as an add‑on for those who can’t tolerate cytomel spikes?

There’s no published pharmacokinetic evidence that compounded T3 achieves slow release, and accurately dosing microgram quantities is technically tricky, so I don’t recommend it.

So the practical path is start with T4, and if a minority don’t feel right or stay off biochemically, add T3, with desiccated sometimes being better tolerated.

I begin with levothyroxine but treat hypothyroidism as a higher cardiometabolic‑risk state, address confounders like menopause, then consider combination therapy; I like synthetic combos because you can set the T4:T3 ratio, and data point to around three‑and‑a‑half to one.

For desiccated brands and quality, patients can check FDA recalls, and recalls affect both desiccated and levothyroxine products.

On brands versus generics, studies show no meaningful difference; pharmacies often switch patients without notice, and past brand preference was largely marketing despite a lack of evidence.

Levothyroxine potency is tightly controlled now at plus or minus five percent across shelf life, which matters because small shifts can have clinical effects.

What should be the therapeutic target—biochemistry or symptoms?

Guidelines aim for biochemical euthyroidism—normal TSH with a reasonable free T4—and de‑emphasize symptoms, but if patients still feel unwell, I consider that an incomplete response and support trying combination therapy.

Case one: very high TSH that won’t normalize without hyperthyroid symptoms; how do you manage that?

I’d suspect assay interference from heterophile antibodies or aggregated TSH, confirm with alternative testing if possible, and in stubborn cases guide therapy by free T4 and the clinical picture rather than TSH.

Case two: a patient whose TSH swings from fully suppressed to elevated with tiny dose changes.

When pituitary feedback is extremely sensitive, manage to a normal free T4 and accept a suppressed TSH if the patient is clinically well, much like we do in select pregnancy scenarios.

On iodine, some take very high doses or avoid iodized salt altogether; what’s the risk?

Excess iodine can trigger autoimmune thyroid disease and, in nodular glands, iodine‑induced hyperthyroidism; adults need about 150 micrograms daily, and high habitual intake, like in parts of Japan, correlates with more autoimmunity.

Any male‑female differences beyond the symptom overlap you mentioned?

Women are affected about ten times more often, postpartum thyroiditis is a distinct entity, and female thyroid tissue may leak more antigen, possibly influenced by sex hormones, though we lack a definitive explanation.

Subclinical case: a forty‑year‑old with TSH around eight to nine, normal free T4, and no symptoms; treat or watch?

I’d repeat labs, assess family history, antibodies, and thyroid ultrasound; if signs point to evolving Hashimoto’s or if metabolic markers benefit, I favor treatment, but in older adults we relax TSH thresholds as the upper limit rises by about one point per decade after fifty.

So a seventy‑year‑old with a TSH of six could be entirely normal, and treatment may do more harm than good.

Exactly; don’t rush to treat older patients based on a modestly elevated TSH alone.

Looking ahead ten years, where do we most need progress—diagnosis or therapy?

Treatment needs to improve; we need robust mass‑spec assays for T3 and a true slow‑release T3 so physicians can restore tissue thyroid status with confidence, and there are promising approaches like T3 polymers and T3 sulfate that may fit a faster 505(b)(2) path.

And we need labs to step up with a reliable free T3 assay; for listeners, Tony’s book Rethinking Hypothyroidism makes a measured case between the extremes and is useful for patients and clinicians alike.

It’s been a pleasure; I’m grateful to help patients, and I hear daily from people who improved on combination therapy after learning more and working with their doctors.

Thanks for joining us, Tony. For show notes and more, visit the site or find me on social, and please remember this podcast is informational only and not medical advice; see our disclosures page for details.