Thyroid Levels, Depression, and Your Serotonin Levels

Hi, I’m Dr. Hagmeyer, and one of the most commonly overlooked areas when we talk about thyroid function and performance is the role serotonin plays.

If you suffer from thyroid problems, you might be all too familiar with anxiety and depression.

In fact, the most common medication men and women are put on after thyroid medication is an antidepressant or an antianxiety medication.

I want to be very clear that these medications DON’T fix your problem, but rather serve as a band-aid, and what you might not know is that if you are taking antidepressants, realize that none of the antidepressant medications actually raise the quantity of the neurotransmitter that the medication is targeting. They only allow the neurotransmitter to remain in the synaptic cleft for a longer period of time.

Here is an illustration of what I’m talking about (see video). You can see the Neurotransmitters (little yellow dots), and the synaptic cleft is the space between the nerves. Antidepressants only allow the Neurotransmitters to stay in this space longer. They do not affect the quantity of Neurotransmitters produced by the nerve.

Low Thyroid Hormones and Structural Changes to the Brain

When a person has low thyroid hormones, not just a low TSH but low T3 levels, low T4 levels, several things happen to the brain.

#1 The Serotonin receptor sites become less sensitive to Serotonin– meaning they don’t respond
#2 There is a decrease in the number of serotonin receptor sites- if there is less receptor sites, there is less places that serotonin can bind to and consequently lower serotonin levels. Also, less serotonin is then produced in the brain and in the other tissues of the body.

Studies show that if you continue to have low Thyroid levels year after year, permanent neurodegeneration of serotonergic neurons in the frontal cortex occurs(*)

Why T3 Is Relevant To Brain Health

T3 is the active thyroid hormone inside many tissues, including the brain. Thyroid hormone signaling supports things like synapse function, myelination, energy metabolism, and aspects of neurogenesis/glial biology—especially in development, but also in adult brain maintenance(*).

 Symptoms of Neurodegeneration of the Brain include:

• Feeling overwhelmed with ideas to manage,
• Feeling of inner rage, feeling of paranoia, not enjoying life
• Lack of enthusiasm,
• Loss of interest in friendships,
• Feeling of  anger,
• Difficulty falling into a restful state of sleep.

Again, these are all classic Symptoms that the frontal cortex of your brain and the Serotonergic pathways are shutting down.

Traditional medicine typically prescribes SSRIs for these kinds of symptoms- medications like Paxil, Zoloft, Celexa, Lexipro and too many others to list.

Here’s a study showing the effects of Low Thyroid or hypothyroidism on the Serotonin pathways of the brain(*),(*).

If you do suffer from these symptoms, you are probably wondering is it too late?, and if not, what can be done to support my brain and consequently my thyroid.

#1 Many natural compounds can actually increase the quantity of serotonin. Remember, medications can’t do this; they only keep the Neurotransmitters around in the synaptic cleft for a longer period of time, they don’t increase neurotransmitter production or neurotransmitter receptor binding.

#2 Supporting NT and the brain requires understanding what the nerve needs metabolically and neurologically.

How Low Thyroid Function Can Cause Depression

So here are three common ways low thyroid function can drive depression-like symptoms

1) Reduced serotonin signaling in the brain

T3 influences how sensitive serotonin receptors are and how strongly serotonin “lands” in the brain. When T3 is low, many people experience:

• low mood, apathy, reduced pleasure

• brain fog, slower processing

• more negative rumination

• social withdrawal

This is one reason some people don’t fully respond to typical antidepressant approaches until thyroid function is optimized.

2) Slowed conversion of T4 → T3

Many patients are told, “Your TSH is normal, so your thyroid is fine.” But mood symptoms often track closer with T3 availability than with TSH alone.

Common reasons T4-to-T3 conversion drops:

• chronic stress / elevated cortisol

• inflammation (including autoimmunity)

• calorie restriction / under-eating

• low iron, selenium, zinc

• insulin resistance

• chronic illness

If your body isn’t converting well, you can have “normal” T4 while your cells—and your brain—are operating in a low-T3 state.

Read: How Low Iron Shuts Down the Thyroid

3) Inflammation shifts tryptophan away from serotonin

Serotonin is made from tryptophan. But when inflammation is high, tryptophan is often diverted down inflammatory pathways (instead of becoming serotonin). In functional medicine, this matters because thyroid dysfunction and inflammation often travel together (especially in Hashimoto’s).

So the bloodwork pattern can look like:

• thyroid antibodies/inflammation ↑

• serotonin production and signaling ↓

• depression, anxiety, insomnia ↑

Watch this video: How Inflammation Causes Low Thyroid

Support Your Brain When You Have Low T3

• Supporting circulation and blood flow to the brain
• Supplements that will help with the uptake of NT into the synaptic cleft
• Supporting amino acid precursors- certain amino acids, such as 5-HTP or SAMe, can help increase the quantity of these Neurotransmitters
• Nutritional cofactors that are needed to make the Neurotransmitter- things like B6, B12, Folate, magnesium, niacin
• blood sugar stability
• reducing Inflammation
• supporting optimal Gut health
• Phosphatidyl Serine

Read: Naturally Boost Serotonin Levels- Without Medication

Here’s what You Should Remember About Low Thyroid and Depression

1. Low thyroid isn’t just a “metabolism problem”—it can be a brain and mental health problem, too.
   One of the most overlooked truths in clinical practice is that low thyroid and mental health are deeply connected. Your brain is one of the most thyroid-sensitive organs in your body. When thyroid function is low—especially when T3 is low—your brain may not get the stimulation it needs to regulate mood, motivation, focus, and emotional resilience.
   And that’s why so many people experience low thyroid and depression symptoms that are dismissed as “just stress” or “just anxiety,” when in reality the physiology is contributing.
2. Low T3 and depression often go together—because T3 is the brain’s “active” thyroid hormone.
   T4 is largely a storage hormone. T3 is the active hormone your cells actually use—and your brain is heavily dependent on it. In many patients, standard testing can look “fine,” yet the person still struggles with low T3 and depression symptoms, such as:

• low mood, irritability, or feeling emotionally flat
• brain fog, poor memory, slow thinking
• low motivation, low drive, low confidence
• anxiety, overwhelm, or feeling more reactive than normal
• sleep disruption, fatigue that doesn’t lift
  This is part of the low thyroid effect on the brain that doesn’t get discussed enough. If your brain can’t access adequate thyroid signaling, mental health symptoms can become persistent—even when someone is “on thyroid medication.”

3. Thyroid disease requires investigation—because replacing hormone alone doesn’t always fix low thyroid and depression.
   Here’s the problem with the old model: it assumes the solution is simply “give thyroid hormone and move on.” But if you’re still dealing with low thyroid and depression, the real question becomes: why is your body not producing, converting, absorbing, or utilizing thyroid hormone effectively—especially T3?
   A functional approach looks at what may be driving the pattern, such as:

• poor conversion of T4 to T3 (stress physiology, inflammation, nutrient deficiencies)
• autoimmune thyroid disease (like Hashimoto’s) and immune-driven inflammation
• gut dysfunction affecting absorption and immune activation
• blood sugar instability, insulin resistance, or chronic inflammatory load
• hormone imbalances (estrogen/progesterone shifts) influencing thyroid signaling
  When those drivers aren’t addressed, patients can stay stuck in the cycle of low energy, brain fog, and low thyroid and mental health symptoms even while taking “the right medication.”

4. There is no cookie-cutter plan I can responsibly give you over email—because low thyroid affects the brain differently in different people.
   Two people can both say they’re dealing with low thyroid and depression, and yet the root causes can be completely different. One may have Hashimoto’s-driven inflammation. Another may have poor conversion to T3. Another may have gut issues impairing absorption. Another may have chronic stress physiology shutting down thyroid signaling.
   That’s why I can’t ethically say, “Take this supplement,” or “Use this dose,” without a full understanding of your case. When we’re talking about low thyroid effect on the brain, details matter—and personalized investigation matters even more.
5. And I’ll end with something important: the longer low thyroid impacts the brain, the more urgent it becomes to address.
   I don’t say this to scare you—I say it because I care about outcomes. In some cases, long-standing thyroid dysfunction can contribute to deeper issues with cognition, mood regulation, and overall neurological resilience. The earlier you address the drivers behind low thyroid and mental health symptoms, the better your chance of improving energy, clarity, and emotional stability.
   So if you’ve been struggling with low T3 and depression, or you feel like your brain simply isn’t working the way it used to, don’t ignore it—investigate it.
6. I’m Dr. Hagmeyer, and I hope today’s video helped you connect the dots.
   If you take one thing away, let it be this: low thyroid and depression is not “all in your head.” It’s often a sign that your brain isn’t getting the thyroid signaling it needs—and fixing it requires a clinician who will partner with you, look deeper, and help you address root causes instead of chasing symptoms.

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