Subclinical Hyperthyroidism: Heart Risks and When to Treat

Most people don’t know they have subclinical hyperthyroidism until it shows up on a routine blood test. No weight loss. No jittery hands. No racing heart. Just a low TSH level-below 0.45 mIU/L-with everything else normal. It’s silent. But that doesn’t mean it’s harmless. Especially for older adults. Especially for the heart.

What Exactly Is Subclinical Hyperthyroidism?

Subclinical hyperthyroidism means your thyroid is working a little too hard, but not enough to push your T4 or T3 into the high range. Your TSH-the hormone your brain sends to tell your thyroid to slow down-is suppressed. That’s the only sign. It’s like your car’s check engine light is on, but the engine still runs fine. Only this time, the warning isn’t about fuel or oil. It’s about your heart.

It’s most common in people over 65. One study found nearly 1 in 6 adults over 75 have it. Often, it’s caused by a toxic nodular goiter-a few overactive lumps in the thyroid-or by taking too much thyroid medication after being treated for hypothyroidism. Less often, it’s from Graves’ disease. The key is that it’s persistent. One low TSH reading doesn’t count. Doctors look for at least two or three low readings over months to confirm it’s real.

Why the Heart Is the Main Concern

The heart doesn’t like being pushed too hard, even gently. Subclinical hyperthyroidism doesn’t make your heart race all day like overt hyperthyroidism does. But it changes how it beats, how it relaxes, and how long it takes to recover between beats.

Studies show people with TSH below 0.1 mIU/L have more than double the risk of atrial fibrillation-a chaotic, irregular heartbeat that can lead to stroke. One analysis of over 8,700 people found those with TSH this low had a 2.5 times higher chance of developing AFib. Even those with TSH between 0.1 and 0.44 mIU/L had a 63% higher risk. That’s not small.

Heart failure risk also climbs. A 10-year study tracking nearly 25,400 people found those with TSH under 0.1 had almost twice the risk of heart failure compared to people with normal thyroid function. In people over 60, the risk of atrial fibrillation triples over a decade. Left ventricular mass increases. The heart muscle thickens. Diastolic function-the heart’s ability to fill with blood between beats-gets worse. Heart rate variability drops, meaning the nervous system’s control over the heart is weakening.

These changes don’t always cause symptoms. That’s why they’re dangerous. Someone might feel fine, go about their life, and suddenly have a stroke or need a pacemaker because their heart’s been quietly damaged for years.

Bones and Brain: The Hidden Costs

The heart isn’t the only thing at risk. Bone density takes a hit. When TSH drops below 0.1, the risk of fractures-especially hip and spine fractures-goes up by more than double. This is especially worrying for postmenopausal women and older men who already have lower bone mass.

Cognitive effects are less clear, but some studies point to subtle declines in memory and executive function-things like planning, organizing, and switching tasks-in older adults with long-term subclinical hyperthyroidism. It’s not dementia. But it’s enough to make daily life harder.

Quality of life? Usually fine-if you don’t have symptoms. But if you start feeling tired, anxious, or notice your heart skipping beats, those subtle changes start to matter more.

When Should You Treat It?

Not everyone with subclinical hyperthyroidism needs treatment. That’s the big debate. But the guidelines are clear: risk matters more than numbers alone.

If your TSH is below 0.1 mIU/L and you’re over 65, most experts say treat it. Especially if you have high blood pressure, heart disease, osteoporosis, or a family history of atrial fibrillation. Even if you feel fine. The damage is already happening.

If your TSH is between 0.1 and 0.44 mIU/L, treatment isn’t automatic. You need other red flags: symptoms like palpitations, confirmed bone loss, or signs of heart strain on an echocardiogram. If you’re young, healthy, and have no heart issues, doctors might just watch and wait.

First-line treatment? Beta-blockers. They don’t fix the thyroid. But they calm the heart. They lower heart rate, reduce palpitations, and can even reverse some of the thickening of the heart muscle. They’re safe, well-tolerated, and give you time to decide on the next step.

If the cause is a toxic nodule or Graves’ disease, you’ll need to target the thyroid itself. Radioactive iodine is the most common option-it shrinks the overactive tissue. Surgery is less common but used if the thyroid is very large or if you can’t take iodine. The goal isn’t to cure it completely. It’s to bring TSH back into the normal range, not to push it into hypothyroidism.

That’s the tightrope. Treat too aggressively, and you create a new problem: hypothyroidism. That brings its own risks-higher cholesterol, fatigue, weight gain, and even more heart strain. The goal is balance, not perfection.

How Often Should You Be Monitored?

Monitoring isn’t optional. It’s the cornerstone of managing this condition.

• If TSH is below 0.1 mIU/L: Check every 3 to 6 months. Get an ECG and bone density scan if you’re over 65.
• If TSH is between 0.1 and 0.44 mIU/L and you’re low-risk: Check once a year.
• If you’re on beta-blockers: Monitor every 3 months until stable, then every 6 months.
• If you’re on thyroid treatment (like radioactive iodine): Monitor closely for the first 6 months to avoid overshooting into hypothyroidism.

Don’t forget your doctor should also check your heart. An echocardiogram every 1 to 2 years is wise if your TSH is persistently low. A bone density test (DEXA scan) is recommended for women over 65 and men over 70 with suppressed TSH.

What’s Changing in 2026?

Guidelines are shifting. The American Thyroid Association’s 2016 advice was conservative. But newer data is pushing doctors to act sooner.

The DEPOSIT study, tracking 5,000 older adults across Europe, is wrapping up in 2026. Early results suggest even mild TSH suppression (0.1-0.44) increases heart risk in people with existing heart disease. That could change who gets treated.

The THAMES trial, led by UCLA, is comparing outcomes in patients with TSH under 0.1 who are treated versus those who aren’t. Results expected in late 2024 could finally give us randomized proof that treating early saves lives.

The American Heart Association is calling for more trials. They’re tired of guessing. They want hard data to answer one question: Does treating subclinical hyperthyroidism reduce heart attacks, strokes, and deaths?

Right now, the answer is: probably yes-for high-risk people. But we’re getting closer to knowing exactly who benefits most.

What Should You Do Now?

If you’ve been told you have subclinical hyperthyroidism:

• Don’t panic. You’re not alone. Millions have it.
• Don’t ignore it. Especially if you’re over 65 or have heart disease.
• Ask for an ECG and an echocardiogram.
• Ask about a bone density scan if you’re a woman over 65 or a man over 70.
• Ask if beta-blockers make sense-even if you feel fine.
• Ask why your TSH is low. Is it from medication? A nodule? Graves’?
• Don’t rush into radioactive iodine unless you’re high-risk.

It’s not about fixing a number. It’s about protecting your heart, your bones, and your future. Subclinical hyperthyroidism doesn’t roar. It whispers. But if you listen, you can stop it before it breaks something.