Is a Sluggish Thyroid Behind Your Fatigue & Unexplained Weight Gain?

Jaime Heer, FNTP, RWS
Jul 9
6 min read

Plus: The Full Thyroid Lab Panel Your Doctor Should Be Running, and How to Support Your Thyroid Naturally

What is the thyroid?

Your thyroid is a small, butterfly-shaped gland that sits on the front of your neck, right over your trachea. Despite its size, this little gland plays a huge role in your overall health. It produces hormones—primarily T4 (thyroxine) and T3 (triiodothyronine)—that regulate metabolism, energy production, body temperature, and even digestion and mood.

Every single cell in your body depends on thyroid hormones to burn calories and oxygen to create energy. So when your thyroid slows down (aka becomes sluggish), it’s no surprise that fatigue and weight gain often follow.

But here’s the thing: low thyroid function is rarely just a thyroid problem. It’s often the downstream result of stress, inflammation, nutrient deficiencies, gut imbalances, and poor blood sugar control.

Let’s take a look at how a healthy thyroid should function—and what might be disrupting yours.

How a Healthy Thyroid Works: Your HPT Axis

Your thyroid doesn’t work alone. It’s part of a larger system called the HPT Axis—short for Hypothalamus-Pituitary-Thyroid Axis—which is how your brain communicates with your thyroid gland to regulate hormone production.

Here’s a simplified version of how it works:

Hypothalamus (in the brain) detects low thyroid hormones in your blood and signals the pituitary gland to release TSH (Thyroid Stimulating Hormone).
TSH tells the thyroid gland to produce T4, which is then converted—mostly in the liver and gut—into the active form T3.
T3 is the hormone that actually increases your metabolism, regulates body temperature, and generates energy.
Once enough thyroid hormone is circulating, the brain slows down TSH production, keeping everything in balance.

A helpful analogy:

Pituitary gland = Thermostat
Thyroid gland = Furnace
T3/T4 = Heat

If something interferes with this system—nutrient deficiencies, inflammation, stress, or poor gut health—it’s like someone messing with the wires on your thermostat. The system breaks down, and you end up with too little T3… and too many symptoms.

Symptoms of a Sluggish Thyroid

If your thyroid isn’t producing or converting enough hormone, you may experience:

Persistent fatigue
Difficulty losing weight (despite diet & exercise)
Hair thinning or hair loss
Constipation
Dry skin
Feeling cold often
Brain fog or poor memory
Depression or low motivation
Irregular periods or fertility struggles
Loss of the outer third of the eyebrows
High cholesterol
Muscle aches and stiffness

What Causes Low Thyroid Function?

Several systems in the body influence thyroid function. If even one of these processes is off, the entire system can slow down.

1. Chronic Stress

Stress raises cortisol, your body’s primary stress hormone. High cortisol suppresses TSH and slows the conversion of T4 into active T3.
Chronic stress can also cause the body to produce more reverse T3, an inactive form that blocks T3 from doing its job.
Long-term stress dysregulates both your HPA axis (stress response) and HPT axis, leading to sluggish thyroid symptoms.
Stress can even trigger autoimmune thyroid diseases like Hashimoto’s.

2. Poor Blood Sugar Balance

Frequent blood sugar highs and lows cause stress in the body, increasing cortisol and inflammation.
Chronically high insulin (from a high-sugar or refined-carb diet) interferes with thyroid hormone production and conversion.
Insulin resistance can also reduce your cells’ sensitivity to thyroid hormones—even if your blood levels look “normal.”

3. Gut Health

You need a healthy gut to convert T4 into T3 and absorb nutrients like selenium and zinc, which are critical for thyroid hormone production.
An imbalanced microbiome, leaky gut, or poor digestion = fewer nutrients = impaired thyroid function.
Gut inflammation can also contribute to autoimmune thyroid conditions like Hashimoto’s, where your immune system attacks your thyroid tissue.

4. Nutrient Deficiencies

Your thyroid needs certain vitamins and minerals to function:

Key nutrients for thyroid health:

Iodine – builds thyroid hormone
Selenium – supports T4 to T3 conversion
Zinc & Magnesium – co-factors for hormone production
Vitamin A, D, C, and B Vitamins – support energy, immunity, and hormone balance
Tyrosine – an amino acid required to make T4

Why Your Labs Might Look “Normal” (But You Feel Terrible)

If you’ve been told your thyroid labs are “normal,” but you still feel tired, foggy, cold, or stuck with stubborn weight gain—you’re not imagining things.

Most conventional doctors only test TSH and maybe Free T4 when assessing thyroid function. But these markers alone don’t give the full picture. You can have a perfectly “normal” TSH and still have low active thyroid hormone (T3), poor conversion of T4 to T3, or even an autoimmune thyroid condition like Hashimoto’s—all of which can cause symptoms.

To make things more confusing, conventional lab ranges are based on population averages (including people who are already unwell). That means something can fall in the “normal” range, even if it’s far from ideal for vibrant energy, metabolism, and hormone balance.

Most conventional doctors only run TSH and sometimes Free T4. But that’s not enough to get the full picture of your thyroid health.

A normal TSH doesn’t guarantee that your body is converting T4 into usable T3—or that your cells are responding to thyroid hormone correctly.

Optimal vs. Normal Thyroid Lab Ranges

Functional medicine practitioners look for optimal ranges—the tighter, symptom-free sweet spots where your body performs best.

Here’s a side-by-side comparison to help you see the difference:

Marker	Conventional Range	Functional/Optimal Range
TSH	0.45 – 4.5 mIU/mL	1.0 – 2.0 mIU/mL
Free T4	0.7 – 2.4 ng/dL	1.0 – 1.5 ng/dL
Free T3	2.3 – 4.8 pg/mL	3.0 – 3.25 pg/mL
Reverse T3	9.2 – 24.1 ng/dL	14.9-26.7 ng/dL
Total T4	4.5 -12.5 µg/dL	7.5-8.1 µg/dL
Total T3	80 – 230 ng/dL	90 – 168 ng/dL
T3 Uptake	24 -39%	27 - 37%
Thyroxine Binding Globulin	12.7 - 30.9 µg/dL	18 - 27 µg/dL
TPO Antibodies	Negative	Negative
Thyroglobulin Antibodies	Negative	Negative

Why does this matter?

Let’s say your TSH is 3.5 µIU/mL. Your doctor may tell you it’s “normal,” but you still feel exhausted, cold, and unable to lose weight. From a functional perspective, a TSH that high could signal that your thyroid is struggling—especially if Free T3 is low.

A higher TSH often means your body is shouting at your thyroid to produce more hormone because there isn’t enough T4 and T3 circulating in your bloodstream. It's like your brain keeps turning up the thermostat because the “heat” (thyroid hormone) just isn’t getting the job done.

Similarly, if your Free T3 is 2.5 pg/mL, that’s “normal,” but far below optimal for most people to feel energized and mentally clear.

This is why it’s essential to go beyond basic lab work—especially if you're experiencing symptoms.

Ask Your Doctor (or Practitioner) for a Full Thyroid Panel

To get a clear view of your thyroid health, you need a comprehensive panel. Here are the markers I recommend at a minimum:

Minimum Thyroid Markers to Ask For:

TSH
Free T4
Free T3
Total T4
Total T3
Reverse T3

To check for autoimmune thyroid disease (like Hashimoto’s or Graves’):

Thyroid Peroxidase Antibodies (TPO Ab)
Thyroglobulin Antibodies (Tg Ab)

Additional Helpful Markers:

T3 Uptake
Thyroxine Binding Globulin (TBG)

Also Consider:

To identify why your thyroid may be under-functioning, these labs can be incredibly helpful:

Cortisol (AM or saliva pattern) – to evaluate your stress response
Fasting Insulin & Hemoglobin A1C – for blood sugar and insulin resistance
GI MAP (stool test) – to assess gut health, inflammation, infections, and microbial balance

How to Support Your Thyroid Naturally

If you're dealing with a sluggish thyroid—or suspect you might be—there’s good news. There’s a lot you can do through diet and lifestyle to support your thyroid and feel better.

Diet Tips:

Eat a whole-food, anti-inflammatory diet rich in protein, healthy fats, and colorful vegetables
Include selenium-rich foods (like Brazil nuts, sardines, and pastured eggs)
Choose iodine sources like seaweed, wild fish, or iodine-containing salt (unless you have Hashimoto’s—then this may need to be individualized)
Support your gut with fermented foods and high-quality probiotics
Avoid ultra-processed foods, refined sugars, and industrial seed oils that cause inflammation
Avoid gluten as this can be a trigger for inflammation & autoimmune conditions like Hashimoto's.

Lifestyle Tips:

Prioritize stress management (breath work, walks in nature, journaling, yoga)
Get adequate, high-quality sleep to restore your HPA-HPT axis
Minimize exposure to endocrine disruptors in personal care and cleaning products
Move your body daily to improve blood sugar regulation and hormone sensitivity
Work with a qualified functional practitioner to personalize a protocol

If you’re struggling with fatigue, weight gain, mood changes, or hair loss—and your labs keep coming back “normal”—you deserve a deeper look.

Getting the right labs, understanding your root causes, and making simple yet strategic changes to your diet and lifestyle can help you reclaim your energy, boost metabolism, and feel like yourself again.

To uncover & address root causes, book a 1:1 session with me! Click the button below to learn more about working together.

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