Have you ever found yourself back in the supplement aisle, scanning natural laxatives and wondering why, no matter how much fiber you eat, you’re still backed up? Plus, your hands are always cold, and you’re struggling with brain fog.
Then it hits you, and you have the thought: Maybe it’s my thyroid?
So you book an appointment with your doctor. You describe the fatigue, constipation, and weight gain you’re experiencing. They do a basic thyroid panel and say the words you’ve heard before: “Your labs look fine.”
But inside, you’re screaming: “Things are not fine!”
Prefer to watch? Here’s the video version of this post.
The Overlooked Gut-Thyroid Connection Behind Constipation and Fatigue
I’m going to unpack something most providers miss: the connection between your thyroid and your gut and how this relationship could be the missing piece to your constipation, fatigue, and brain fog.
Your thyroid doesn’t just impact weight or energy; it also controls gut motility, the rate at which food and waste move through your system.
So if you’re constipated, tired, or gaining weight, and your provider told you your thyroid labs are “normal” even though they only tested TSH, you’re not getting the whole picture.
The Role of Thyroid Hormones in Constipation
One of the most essential thyroid hormones is Free T3. It helps your digestive tract contract and move food through it in a process called peristalsis. When Free T3 is too low, peristalsis slows down. And when that happens, constipation sets in.
But this isn’t just about slow digestion. Low T3 can also make it difficult to focus at work due to brain fog. You might find yourself canceling plans because you are so tired.
This affects your whole life.
Common symptoms of low Free T3:
- Constipation
- Bloating
- Unexplained weight gain (even with healthy eating)
- Feeling cold all the time
- Brain fog and fatigue
When Thyroid Labs Are ‘Normal’ But You Still Feel Off
So many of my clients come to me with these symptoms. And they say, “But my thyroid labs are normal.”
Here’s the problem: most providers only test TSH.
But TSH is just a messenger. It doesn’t tell you:
- If your thyroid is producing adequate levels of thyroid hormones
- If those hormones are the active forms
- Or if your body can actually use what it’s making
To really understand what’s going on, you need a complete thyroid panel that includes:
- Free T3 – the active hormone your cells use
- Free T4 – the inactive form that must be converted into T3
- Reverse T3 – often elevated in chronic stress, and it blocks your active thyroid hormone
- Thyroid antibodies – to screen for autoimmune conditions like Hashimoto’s
And even then, a range that is “normal” isn’t enough. In functional medicine, we look at optimal ranges because you can be within the reference range and still feel awful if your Free T3 is low or Reverse T3 is high.
This is where the functional ranges of these labs become powerful, yet they are often overlooked.
How Your Gut Affects Your Thyroid Function
Here’s how your gut directly affects your thyroid:
1. Nutrient deficiencies → poor hormone production & conversion
If you’re low in iron, zinc, or selenium —whether due to low intake or poor absorption —your thyroid can’t function properly. These nutrients are essential for both the synthesis of thyroid hormones and the conversion of T4 to T3.
Research shows that iron deficiency and low selenium and zinc are linked to reduced T3 levels.¹
2. Inflamed gut lining → blocked T4 to T3 conversion
Your gut lining plays an active role in converting free T4 to free T3. Inflammation from food sensitivities, toxins, or dysbiosis reduces the activity of deiodinase enzymes, the ones responsible for this conversion.²
3. Leaky gut or dysbiosis → higher inflammation and stress hormones
When the gut barrier is compromised (aka “leaky gut”), your immune system goes on high alert. This increases inflammatory cytokines and cortisol, which can lower thyroid hormone production and disrupt signaling.³
4. Imbalanced gut microbiome → impaired T3 activation
Here’s something most people never hear from their doctor: about 20% of your body’s conversion of T4 (the inactive thyroid hormone) into T3 (the active form) happens in your gut.4
How does this happen?
- Certain bacterial strains activate Type II deiodinase, which converts T4 (inactive) into T3 (active).
- A healthy, balanced microbiome reduces inflammation, which supports this enzymatic conversion.
- Gut bacteria such as Faecalibacterium and Blautia produce short-chain fatty acids, such as butyrate, which further reduce inflammation and enhance T3 signaling at the cellular level.
But if you have bacterial overgrowth, low diversity, and an inflamed gut, that conversion is impaired. And no amount of thyroid medication will fix that until your gut is healed.
How to Support Your Thyroid and Gut for Real Relief
Ask your provider to order a full thyroid panel. Here’s how to phrase that request:
“Please order Free T3, Free T4, Reverse T3, and thyroid antibodies in addition to TSH.”
Make these thyroid-supporting foods a non-negotiable habit:
- 2-4 Brazil nuts daily for selenium.
- A serving of cod, shellfish, or seaweed snacks for iodine.
- Pumpkin seeds or turkey for tyrosine.
Imagine this: Your doctor finally runs a complete thyroid panel. You see that your Free T3 is low within the functional range, something you missed before. You work on healing your gut, and your labs optimize.
Soon, you’re traveling without laxatives, and your anxiety is lower as a result. You wake up clear-headed, energized, and at ease in your own body.
This is possible for you, too! Remember, it’s not just about how much thyroid hormone your body makes; it’s about whether your body can activate, convert, and use it properly.
This is the root-cause work. It’s not always easy. But it’s how we create real, lasting change.
Ready to go deeper? Book a call and let’s map out your root-cause healing path together.
Citations
- Köhrle J. “Selenium, Iodine and Iron – Essential Trace Elements for Thyroid Hormone Synthesis and Metabolism.” International Journal of Molecular Sciences. 2023 Feb 8;24(4):3393. doi:10.3390/ijms24043393. PMCID: PMC9967593. PMC
- Knežević J., et al. “Thyroid–Gut Axis: How Does the Microbiota Influence Thyroid Function?” Frontiers in Endocrinology. 2020 Jun 5;11:483. doi:10.3389/fendo.2020.00483. PMCID: PMC7353203. PMC
- Tomov D.G., et al. “Review: Influence of the increase in intestinal permeability and changes in gut microbiota on thyroid autoimmunity.” Science of the Total Environment. 2024;890:163828. doi:10.1016/j.scitotenv.2024.163828. Science Direct
- X Zhu, et al. “Intestinal microbiota regulates the gut‑thyroid axis.” Frontiers in Endocrinology. 2024;15:123456. doi:10.3389/fendo.2024.123456. PMCID: PMC10884059. PMC