Your Thyroid — What It Actually Does, When It's Really the Problem, and When It's Not

YOUR HEALTH — Issue No. 13

"My thyroid is off."

"I think I am having thyroid problems."

"It must be my thyroid."

I hear some version of this every week in the exam room. And I understand why. Fatigue, weight gain, hair loss, brain fog, depression, constipation — these symptoms are real, they are disruptive, and people want an answer. The thyroid has become that answer for a lot of people.

The truth is more complicated.

Thyroid disease is genuinely common — it is one of the most prevalent endocrine conditions in medicine, affecting millions of Americans, the majority of them women. I am not dismissing it. There are patients whose thyroid disease is complex, difficult to manage, and genuinely life-altering. Getting some patients to a stable euthyroid state is one of the more challenging things we do in primary care.

But thyroid disease is also one of the most over-diagnosed and over-treated conditions in medicine. And that matters — because the treatment itself carries real risks when it is not needed.

This issue is about both of those realities. The thyroid is important. It also gets blamed for a lot of things it did not cause. Understanding the difference is what this issue is for.

What your thyroid actually does

The thyroid is a small butterfly-shaped gland in the front of your neck, just below your Adams apple. It produces two primary hormones — thyroxine (T4) and triiodothyronine (T3) — that regulate some of the most fundamental processes in your body.

When your thyroid is working correctly you generally have no idea it exists. When it is not — the effects are wide-ranging because thyroid hormone touches nearly every organ system.

The primary functions of thyroid hormone:

Metabolism — thyroid hormone regulates how quickly your body converts food into energy. Too little and metabolism slows — leading to weight gain, fatigue, and feeling cold. Too much and metabolism accelerates — leading to weight loss, heat intolerance, and restlessness.

Energy production — thyroid hormone is essential for mitochondrial function — the process by which your cells generate energy. This is why fatigue is one of the most common symptoms of both low and high thyroid function.

Heart rate and cardiovascular function — thyroid hormone directly affects heart rate and cardiac output. Hypothyroidism slows the heart. Hyperthyroidism speeds it up and can trigger arrhythmias.

Gut motility — thyroid hormone regulates how quickly food moves through the digestive tract. Low thyroid function slows gut motility — contributing to constipation, bloating, and digestive discomfort.

Bone turnover — thyroid hormone plays a role in maintaining bone density. Excess thyroid hormone — whether from overactive thyroid disease or over-replacement with medication — accelerates bone loss and increases fracture risk over time.

Brain function and mood — thyroid hormone affects neurotransmitter production and brain metabolism. Both hypothyroidism and hyperthyroidism can affect mood, cognition, and mental clarity.

Hair, skin, and nails — thyroid hormone influences the growth cycle of hair follicles and the health of skin and nails. Hair loss and dry skin are common symptoms of hypothyroidism — though they are also common symptoms of many other conditions.

How TSH works — the thermostat analogy

TSH — thyroid stimulating hormone — is produced by the pituitary gland in the brain, not by the thyroid itself. Think of it as a thermostat.

When thyroid hormone levels drop — the pituitary turns up the signal, producing more TSH to tell the thyroid to work harder. When thyroid hormone levels are adequate — the pituitary pulls back, producing less TSH.

This is why a high TSH suggests the pituitary is working overtime to stimulate an underperforming thyroid — pointing toward hypothyroidism.

And why a low TSH suggests the pituitary has pulled back because the thyroid is producing more than enough — pointing toward hyperthyroidism or over-replacement with medication.

TSH is the best single screening test for thyroid dysfunction — but it is a snapshot, not the complete picture. Context matters enormously.

The brain-gut-thyroid connection

The thyroid does not function in isolation. Two systems that significantly influence thyroid function are often overlooked — the brain and the gut.

The brain connection:
Chronic stress elevates cortisol — the body's primary stress hormone. Sustained high cortisol suppresses the conversion of T4 to the more active T3, reduces pituitary TSH output, and can produce thyroid-like symptoms in a person with a completely normal thyroid. This is one reason stress management is not optional in thyroid health — it is clinically relevant.

The gut connection:
Approximately 20% of T4 to T3 conversion happens in the gut — not the thyroid. Chronic gut inflammation, poor microbiome diversity, or intestinal permeability can impair this conversion — meaning a patient can have adequate T4 production but insufficient active T3 at the cellular level, even with a normal TSH.

Additionally, chronic systemic inflammation — from any source including the gut — independently suppresses thyroid function and can produce abnormal TSH readings in a person whose thyroid itself is functioning normally.

This is why addressing gut health, reducing inflammation, and managing stress are legitimate first steps before attributing symptoms to thyroid dysfunction — and before reaching for a prescription.

Why the thyroid gets blamed too much — the overlapping syndromes problem

Here is the clinical reality that is rarely stated plainly:

The symptoms most commonly attributed to hypothyroidism are almost completely nonspecific. Fatigue. Weight gain. Brain fog. Hair loss. Constipation. Depression. Cold intolerance. Low energy.

Every one of these symptoms is also caused — frequently — by conditions that have nothing to do with the thyroid:

A patient who comes in with fatigue, weight gain, and hair loss has a long differential diagnosis. The thyroid is one item on that list — not the presumptive answer.

When a provider orders a TSH and it comes back mildly elevated — it is tempting to connect the dots. But the honest clinical question is: would this patient's symptoms actually improve with thyroid treatment? The data suggests that in many cases of mild subclinical hypothyroidism — the answer is no.

Subclinical hypothyroidism — what the data actually shows

Subclinical hypothyroidism is defined as a mildly elevated TSH — typically between 4.0 and 10.0 mIU/L — with normal free T4 and free T3 levels and no or minimal symptoms.

This is the most commonly over-treated thyroid condition in medicine.

What the evidence shows:

A meta-analysis found no improvement in quality of life, cognition, blood pressure, or body weight with levothyroxine treatment in subclinical hypothyroidism. A randomized trial of older adults found no relief in hypothyroid symptoms or fatigue with medication treatment. And approximately 60% of patients with mild subclinical hypothyroidism will see their TSH spontaneously normalize without any treatment at all.

The clinical approach:

If your TSH comes back elevated for the first time — the appropriate next step is not an immediate prescription. It is a recheck. At that follow-up visit, ordering TSH alongside free T4 and free T3 gives a clearer, more complete picture of what the thyroid is actually doing — not just what the pituitary is signaling.

Address gut health. Reduce inflammation. Manage stress. Optimize sleep. Then recheck in 6-8 weeks.

If the TSH remains persistently elevated above 10 mIU/L — treatment is indicated regardless of symptoms. If it is between 4.0 and 10.0 — the decision to treat should be individualized based on symptoms, age, cardiovascular risk, and patient preference. Not reflexive.

Something worth saying plainly:

Once a patient is started on levothyroxine, any provider who sees them afterward will assume the diagnosis was made correctly and the medication is necessary. The prescription gets refilled. The dose gets adjusted. Years pass. And it is entirely possible — more common than most people realize — that the thyroid was functioning just fine all along.

This is not a criticism of any individual provider. It is a systems reality. A borderline TSH on a single draw, treated with levothyroxine before a recheck, can become a lifelong prescription that was never truly indicated. That is worth knowing — whether you are the patient or the provider.

The over-treatment problem — and why it matters

Levothyroxine is one of the most commonly prescribed medications in the United States. And over-replacement — meaning a dose that pushes TSH below the normal range — is known to be very common.

Studies using Veterans Health Administration data found that 22-23% of patients on levothyroxine had suppressed TSH levels — meaning they were being over-treated.

This is not a minor issue. The consequences of over-replacement are real and clinically significant.

Atrial fibrillation:
Older patients with suppressed TSH from over-replacement have a greater than five-fold higher likelihood of atrial fibrillation compared to euthyroid patients. Two large population-based studies confirmed increased risk of atrial fibrillation, stroke, and cardiovascular mortality in patients who were over-replaced. The risk is most pronounced when TSH is suppressed below 0.1 mIU/L — but meaningful risk begins below 0.5 mIU/L in older adults.

Bone loss:
Excess thyroid hormone accelerates bone turnover. Long-term over-replacement is associated with increased bone loss and fracture risk — particularly in post-menopausal women and older men. This is one of the most underappreciated consequences of keeping levothyroxine doses too high for too long.

The practical message:
If you are on levothyroxine — your TSH should be checked at least annually. The goal is a TSH in the normal range — not suppressed, not high. If your provider keeps increasing your dose because your symptoms persist despite a normal TSH — that conversation deserves more nuance, not more medication.

The Member deep-dive this week covers the full thyroid test breakdown — TSH, free T4, free T3, total T3, reverse T3, and TPO antibodies — when each is ordered and what each tells us, what to look for when labs are normal but symptoms persist, the gut-thyroid axis in deeper clinical detail, Hashimoto's thyroiditis — the autoimmune mechanism, disease progression, and what patients should know, a brief overview of hyperthyroidism and Graves' disease, and the T3/T4 combination therapy debate.

Next week: Statins — the most prescribed medication in America, the most misunderstood, and why the fear around them may be more dangerous than the medication itself.

Plain Medicine is published for educational purposes only and does not constitute medical advice or establish a patient-provider relationship. Always consult your healthcare provider before making medical decisions.

— Kyle