How Your Thyroid Works, and What Your Lab Results Can Tell You

If you've ever looked at a thyroid lab report and wondered what all the abbreviations mean, you're in good company. The thyroid is one of the more complex hormone systems in the body, and even people who follow their health closely find it confusing. Fortunately, once you understand a few basic ideas, the numbers start to make a lot more sense.

This post walks through how the thyroid works, then explains what each common thyroid test can tell you, along with a few places where the tests fall short.

A Gland That Makes Mostly Inactive Hormone

Your thyroid is a small, butterfly-shaped gland that sits low in the front of your neck. Its job is to pull iodine from your bloodstream and use it to build thyroid hormone.

Surprisingly, most of what the thyroid releases isn't very active. About 80 to 90 percent of its output is T4, a hormone named for the four iodine atoms it carries. T4 doesn't fit well into the receptors on your cells, so on its own it does relatively little.

The hormone that does most of the work is T3. When the body removes one iodine atom from T4, the result is T3, which binds tightly to receptors in nearly every tissue. T3 helps set your heart rate, body temperature, energy use and mental sharpness; you can think of it as the volume knob on your metabolism.

So why would the body rely on an inactive hormone? For most of human history, iodine was hard to come by, and the thyroid system evolved to conserve it. T4 works as a long-lasting reserve with a half-life of about a week, while T3 is used and cleared within about a day.

Each Tissue Controls Its Own Supply

The conversion from T4 to T3 happens through enzymes called deiodinases, and there are three of them:

• D2 is the main converter outside the thyroid. It's especially active in the brain, the pituitary gland, muscle and brown fat.
• D1, found mostly in the liver, kidneys and thyroid, adds to the T3 in circulation and helps clear reverse T3.
• D3 works in the opposite direction; it turns T4 into an inactive form called reverse T3 and breaks down T3 that's no longer needed.

Because of this arrangement, each tissue can turn its own T3 production up or down based on what it needs at the moment, even while blood levels stay steady. It's an elegant system, but it also means a blood test can't tell you everything. A normal level in the blood tells you what's circulating; it doesn't necessarily tell you what your liver or brain is receiving.

TSH: The Signal from the Brain

The thyroid doesn't decide on its own how much hormone to make. Instead, it takes direction from the brain through a feedback loop.

First, the hypothalamus releases a hormone called TRH. TRH prompts the pituitary gland to release thyroid-stimulating hormone, or TSH, which in turn tells the thyroid to produce more T4. When thyroid hormone levels rise, the brain senses the change and eases off on TSH; when they fall, TSH goes up.

It helps to know that TSH itself doesn't cause symptoms. If someone has a very high TSH and feels exhausted, the fatigue comes from the underactive thyroid, and the TSH is simply the alarm signaling the problem.

That's why TSH works so well as a screening test, and also why it's an imperfect guide during treatment. TSH reflects what the pituitary gland sees, and since the pituitary makes its own T3, it can be satisfied while other tissues are still running low. We'll come back to that idea in the treatment post.

Reverse T3: The Body's Brake Pedal

You may have seen reverse T3 described as a hidden sign of thyroid disease. In most cases, the physiology doesn't support that interpretation.

Reverse T3 is inactive, and it doesn't meaningfully block thyroid receptors. It's simply the path T4 takes when the body decides to slow things down. Fasting and serious illness are the classic examples: when food intake drops, the body senses an energy shortage and conserves fuel by making less T3 and more reverse T3. That's a normal, protective response, and the thyroid itself is working as designed.

The Tests on a Thyroid Panel

With that background, the individual tests are much easier to understand. Each one answers a slightly different question, and some are measured more reliably than others.

TSH

TSH is the most sensitive early warning sign. When the thyroid begins to fall behind, TSH usually rises before anything else changes. It's also one of the more reliable tests, especially when you stick with the same lab over time.

Free T4 vs. Total T4

More than 99 percent of the T4 and T3 in your blood is attached to carrier proteins, and only the small unattached, or "free," portion can enter your cells. For that reason, free levels are generally more useful than total levels.

This matters most when estrogen is involved. Pregnancy, oral estrogen and birth control pills all increase carrier proteins, which pushes total T4 up while free T4 stays the same. In those situations, a high total T4 doesn't mean the thyroid is overactive.

Free T4 is the most direct measure of how much hormone the thyroid is producing. The standard test works well for most people, although it can be less reliable in pregnancy, serious illness or conditions that affect carrier proteins. In those cases, a more precise method called equilibrium dialysis with mass spectrometry is available.

Free T3

Because T3 is the active hormone, measuring it seems like the obvious choice. The challenge is accuracy. Standard free T3 tests vary quite a bit, and they tend to be least accurate at low levels, which is exactly the range that matters in hypothyroidism. Mass spectrometry measures low T3 much more accurately, and major reference labs offer it. It's the same reason we measure testosterone by LC/MS; when a decision depends on the low end, the test needs to be accurate at the low end.

T3 also plays a limited role in diagnosis. Early in hypothyroidism, the body works hard to keep T3 normal by raising TSH and drawing down T4, so T3 often stays in range until the condition is well established. It becomes more useful once treatment begins.

Reverse T3

Reverse T3 is even harder to measure precisely, and the same sample can produce quite different results at different labs. If you and your clinician choose to track it, it's best to use one lab consistently and focus on trends rather than a single value.

Thyroid Antibodies

Antibody tests help explain why a thyroid might be struggling. TPO antibodies are the main marker for Hashimoto's thyroiditis, the most common cause of hypothyroidism in the United States, and thyroglobulin antibodies point to the same process, though less reliably. TSH receptor antibodies, on the other hand, are associated with Graves' disease, which causes an overactive thyroid.

Antibodies show that the immune system is involved, but they don't diagnose hypothyroidism on their own. Many people have positive TPO antibodies and a thyroid that continues to work normally for years.

Who Should Be Tested

TSH and free T4 are inexpensive, and having results over several years makes it much easier to spot meaningful changes. At the same time, routine screening in adults with no symptoms and no risk factors hasn't been shown to improve outcomes, and it can lead to overdiagnosis.

Antibody testing is more helpful when there's a family history of thyroid disease, another autoimmune condition, a new heart rhythm problem such as atrial fibrillation or plans for pregnancy.

Preparing for Your Blood Draw

A few simple steps can make your results more accurate:

• Stop biotin for several days before testing. Biotin interferes with many lab tests, and it's common in hair, skin and nail supplements as well as many multivitamins. Ask your clinician how long to hold it for your dose.
• If you take thyroid medication, ask whether to take your dose before or after the blood draw, since timing can affect free T4.
• Use the same lab when you can. Methods and reference ranges differ between labs, and switching makes trends harder to read.
• Keep the full report. A flag in your patient portal only shows that a number fell outside the range, so save the report with the collection date, units and reference range. If you're comparing results from two labs, bring both reports rather than retyped numbers.

Occasionally, a result simply doesn't fit the rest of the picture. Rare antibodies in the blood can interfere with a test and produce a misleading number, so when something doesn't add up, a clinician can check for interference before any treatment decisions are made.

In the next post, we'll look at why symptoms alone can't confirm a thyroid problem and how hypothyroidism is diagnosed.

Asymmetric Health's thyroid care page explains the service available in Lacey and statewide by telehealth. To talk through your symptoms and existing lab reports, contact the clinic.