The Thyroid Labs and What They Mean: An Endocrinologist’s Guide

When patients ask about their thyroid, they often come to me overwhelmed by lab reports and mixed information online. As an endocrinologist who has treated thousands of patients with thyroid disease, I want to simplify this for you, so you can feel informed, empowered, and confident in taking your next steps.

Let’s start with the basics:

What Is the Thyroid Gland?

The thyroid is a small, butterfly-shaped gland located at the base of your neck, just below your Adam’s apple. Though small, it has a big job: it regulates your body's metabolism, energy, and many vital processes.

The thyroid produces three main hormones:

• T4 (Thyroxine): the major hormone produced, later converted into the active form, T3.
  
  

• T3 (Triiodothyronine): the active thyroid hormone that directly affects your cells.
  
  

• Calcitonin: involved in calcium regulation and bone metabolism (a small effect and when the thyroid gland is absent/surgically removed, there is no meaningful impact on calcium and bone health or a deficiency that needs replacement.
  
  

What Is the Role of TSH, Pituitary, and Hypothalamus?

Think of it as a smart control system where the powerhouses are the hypothalamus and pituitary gland:

• The hypothalamus in your brain releases TRH (thyrotropin-releasing hormone).
  
  

• TRH tells the pituitary gland (also in the brain) to release TSH (thyroid-stimulating hormone).
  
  

• TSH then signals the thyroid gland to produce and release T4 and T3.
  
  

If the thyroid is underactive, the pituitary boosts TSH to push the thyroid to work harder. If it's overactive, TSH drops to slow things down: This is how the natural hormonal harmony (homeostasis) is maintained. 

Breaking It Down: Common Questions I Hear from Patients

1. What is a screening test for thyroid disease?

The TSH (Thyroid Stimulating Hormone) blood test is the most common and reliable initial screening. In most cases, when the primary problem is the thyroid gland: 

• If TSH is high, it suggests an underactive thyroid (hypothyroidism).
  
  

• If TSH is low, it suggests an overactive thyroid (hyperthyroidism).
  
  

Most of my patients, I also recommend checking Free T4 at the same time, depending on symptoms, age, gender, women seeking fertility, or those who have any form of neurological condition or are getting evaluated for fatigue. 

As for Free T3, based on guidelines, it is routinely not recommended. I will order it for my patients if any symptoms or if they are on animal thyroid hormone like Armour or NP thyroid for hypothyroidism. Now, I also discuss with my patients that Free T3 assays are not always reliable and not helpful by themselves to make a diagnosis of hypothyroidism or hypothyroidism. 

2. What is Total vs. Free Thyroid Hormone Level?

• Total T4/T3 measures all hormones circulating, both attached to proteins and free in the blood.
  
  

• Free T4 and Free T3 measure the active hormone that is unbound and ready for use by your body.
  Free hormone levels give a more accurate picture of what your body can use. But - in many individuals, if they do not have any reasons to have abnormal protein i.e TBG (thyroid binding globulin), then total T4 can be utilized along with TSH for clinical interpretation. REMEMBER - there are nuances to blood tests. TBG levels are affected by, pregnancy, liver disease, estrogen us,e etc Hence, discuss with an endocrinologist about the abnormal levels.
  
  

3. What are Free T4 and Free T3?

• Free T4 is the storage form and is produced in larger amounts by the thyroid gland as compared to T3— it stays relatively stable and can convert into T3 when needed. Our cells in the body have receptors (doors) where Free T4 attaches, which leads to conversion of Free T4 into Free T3 when the cell needs Free T3. It’s self-regulated. (This is what fascinates me in hormonal pathways)
  
  

• Free T3 is the active form, responsible for energy, metabolism, temperature control, and mood. When Free T3 is very high, as in hyperthyroidism, or if a patient is prescribed high doses of Free T3 or on higher doses of levothyroxine or animal thyroid hormone (eg: armour), the symptoms can be palpitations, anxiety, tremors, sense of doom, sweating etc. The metabolism ramps up.
  
  

In practice, Free T4 is usually more stable. Free T3 can fluctuate more based on stress, illness, and nutritional factors, it is short-lasting.

4. How to Interpret Free T3

When Free T3 is low, and Free T4 and TSH are normal, it can suggest a false lab error or inadequate conversion of T4 to T3, or medication interference.
However, Free T3 should never be interpreted in isolation.
As your endocrinologist, I evaluate the full clinical picture — symptoms, labs, medications, nutrition, supplements, and your entire health history — before recommending treatment.

5. What is Reverse T3 (rT3)? A Simplified Explanation

Reverse T3 is an inactive version of T3. Under stress, severe illness, or low-calorie states, your body can shunt T4 into making more reverse T3 — a protective phenomenon also well known as sick euthyroid syndrome. Now, based on our guidelines, this is a clinical diagnosis, and reverse T3 levels are not necessary to measure. I certainly do not order reverse T3, but I help my patients with optimizing their thyroid hormones and health. At many instances, the protective mechanism of a sick euthyroid helps to heal/recover from an illness. So, supporting with the right supplements, prescriptions, and evaluation helps our patients to get appropriate treatment and solution. 

It's not necessary to measure rT3, but in very complex cases in inpatient/hospitalization settings, it might be checked by the health care team. 

6. I'm Tired — Is It My Thyroid?

Fatigue is one of the most common symptoms of thyroid imbalance, but it’s not the only cause.

• If your TSH, Free T4, and Free T3 are out of range, yes, thyroid could be a factor.
  
  

• If thyroid labs are normal, other causes like sleep apnea, sleep disorders, anemia, vitamin D deficiency, insulin resistance, or inflammation could be involved. If you have hypothyroidism, then your doses might need adjustment to assess if that helps alleviate your fatigue. I will discuss TPO antibodies and autoimmune thyroid disease in another article, but it is a helpful test in individuals.
  
  

This is why a thorough evaluation — not just a lab number — matters. I listen to your full story before jumping to conclusions.

SUBSCRIBE to My Thyroid series on YouTube 

7. My Weight is Higher — Is It My Thyroid?

An underactive thyroid (hypothyroidism) can contribute to weight gain — if left untreated. You can discuss synthetic thyroid hormone (levothyroxine or T4) or Animal thyroid hormone (eg, Armour, NatureThyroid, NP thyroid, etc) or, on certain occasions, a combination of levothyroxine and low-dose liothyronine (T3). It is a shared decision-making process with my patients and me. Both synthetic and animal thyroid hormones have certain calculations and conversion tables for deciding a dose and then titrating based on labs and symptoms. Talk to your physician about persistent weight challenges, as they may usually involve a combination of thyroid dysfunction, insulin resistance, lifestyle factors, and metabolic changes.

An individualized plan that addresses all angles is key, and that's exactly what we create for you in my practice. Remember, hormones are complex; there is a lag period where your thyroid labs will look normal, but your symptoms may not correlate. Hence, seek expertise to get answers to reach your health goals

8. Are There Natural Supplements for Thyroid Balance?

Certain nutrients are crucial for healthy thyroid function:

• Iodine: an essential trace mineral, but excessive supplementation can harm. I would not advise my patients additional iodine supplementation if they have a diagnosis of hyperthyroidism, as it can worsen it. Discuss with your endocrinologist if you need supplementation.
  
  

• Selenium has ways to improve TSH levels and reduce TPO antibodies. It is a trace mineral. ~200 mcg dose has been studied. I usually recommend my patients try it for 60 days and reassess if it helped. After that, they can take 1 Brazil nut a day, which is rich in selenium. WATCH MY VIDEO on Selenium.
  
  

• Vitamin D: Optimize D. It won’t directly treat a thyroid abnormality, but low levels have been linked with autoimmune diseases. And - in general, vitamin D is essential for our bone health, so maintain consistent normal levels.
  
  

Important: Supplements should be personalized. Random use can worsen your health if not guided carefully.

As your endocrinologist, I help you customize a safe and effective plan — combining medical therapy when needed with nutrition, lifestyle, and supplementation.

Final Thoughts: Your Thyroid Health Deserves Expertise

If you are tired of confusion, second-guessing your lab reports, or feeling unheard, you're not alone. Thyroid health is complex — and deserves more than a quick prescription.

📍 At Unified Endocrine & Diabetes Care, I offer a comprehensive thyroid evaluation:

• Personalized lab interpretation
  
  

• Detailed clinical assessment
  
  

• In-house thyroid ultrasound (when needed)
  
  

• Lifestyle, nutrition, and supplement guidance
  
  

✅ Don’t guess when it comes to your health. Schedule a dedicated thyroid consultation with me today.

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Dr. Makhija serves patients in Lafayette, California, and offers care throughout the San Francisco Bay Area, including Walnut Creek, Berkeley, Oakland, San Francisco, Palo Alto, and San Jose. Her personalized approach includes both in-person visits and telemedicine options.

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