“My thyroid was checked, and they told me it was normal.”
I hear this all the time.
Usually, the person sitting across from me is exhausted, cold, constipated, struggling with brain fog, losing hair, gaining weight, or unable to lose weight despite making significant changes to their diet and lifestyle. But because one thyroid test fell inside a laboratory reference range, they were told their thyroid could not possibly be contributing.
Then comes the familiar advice: eat less and exercise more.
If you read Part 1 of this series, you already know I believe calories matter. But metabolism is much more complicated than a calorie calculator, and thyroid hormones are a big part of that equation.
Your Thyroid Is Your Metabolic Thermostat
Your thyroid gland produces primarily thyroxine, or T4, along with smaller amounts of triiodothyronine, or T3.
T4 largely serves as a precursor. Your body converts T4 into T3, the biologically active thyroid hormone responsible for most thyroid hormone action throughout the body.
And T3 matters tremendously when we are talking about weight.
Thyroid hormones help regulate basal metabolic rate, thermogenesis, glucose and lipid metabolism, mitochondrial activity, and energy expenditure. When thyroid hormone activity decreases, the amount of energy your body uses can decrease as well.
This does not mean low thyroid function magically creates body fat. It means the energy side of the energy balance equation can change.
Telling someone to “just create a calorie deficit” without considering the hormones that help regulate energy expenditure is an incomplete conversation.
“But My TSH Is Normal”
Here is where I have a problem with the way thyroid function is often evaluated.
TSH is not even produced by your thyroid. It is produced by your pituitary gland. Under current conventional guidelines, it is commonly used as the primary screening test for diagnosing primary hypothyroidism and hyperthyroidism.
That is very different from saying TSH tells us everything we need to know about thyroid hormone physiology.
It does not.
TSH primarily reflects the feedback relationship between the pituitary and circulating thyroid hormones. It does not directly tell us how much active T3 is available to peripheral tissues, how efficiently T4 is being converted into T3, or what is ultimately happening with thyroid hormone action throughout the body.
Yet I routinely meet patients who were told, “Your TSH is normal, so your thyroid is fine.”
If we are trying to understand why someone is exhausted, cold, constipated, losing hair, gaining weight, and struggling with brain fog, I want considerably more information than a pituitary hormone.
T4 Is Not the End of the Story
Levothyroxine, or synthetic T4, remains the standard treatment for hypothyroidism. For most patients, it is also the only thyroid hormone they are ever offered.
The theory sounds simple: give the body T4 and allow it to convert that T4 into active T3.
But there is an important problem with that assumption.
Your body relies on enzymes called deiodinases to convert T4 into T3. One of the most important is type 2 deiodinase, or D2. Research has shown that higher T4 exposure can cause D2 to become ubiquitinated and inactivated, potentially reducing T4 to T3 conversion in peripheral tissues.
Here is where it gets interesting.
D2 in the hypothalamus and pituitary behaves differently and is less susceptible to this T4 induced inactivation. This means T4 treatment can potentially generate enough local T3 in the hypothalamus and pituitary to suppress TSH while T3 availability elsewhere is not restored to the same degree.
In other words, the TSH can look exactly the way we are told it should while the rest of the thyroid hormone story may be considerably more complicated.
That is why I do not believe thyroid treatment should simply be about giving T4 until TSH lands inside a predetermined range. I care about the active hormone, symptoms, metabolic health, and the individual patient standing in front of me.
What About Thyroid Hormone Resistance?
There are rare genetic disorders called resistance to thyroid hormone. That is not what I am talking about here.
I am talking about the broader concept that thyroid hormone availability and action can differ at the tissue level.
Thyroid hormone has to do more than simply exist in your bloodstream. It has to be transported into cells, converted appropriately, interact with receptors, and ultimately create a cellular response. Deiodinase activity, inflammation, illness, insulin resistance, nutritional status, medications, and other metabolic factors can influence that physiology.
There is still a lot we are learning about tissue specific thyroid hormone action.
But that is exactly why reducing the entire thyroid system to one TSH value makes very little sense to me.
Signs Your Thyroid May Deserve a Closer Look
Thyroid symptoms overlap with many other conditions, so none of these automatically means your thyroid is the problem. But when several occur together, particularly alongside changes in weight or metabolism, they are worth investigating.
Common clues include:
- Difficulty losing weight despite consistent effort
- Unexplained weight gain
- Persistent fatigue
- Feeling unusually cold
- Constipation
- Brain fog
- Hair thinning or increased shedding
- Dry skin
- Reduced exercise tolerance
- Muscle weakness or poor recovery
- Changes in mood or cholesterol
- A personal or family history of thyroid disease
- Persistent symptoms despite being told your thyroid is normal
Iron deficiency, insulin resistance, sleep disorders, sex hormone changes, nutrient deficiencies, autoimmune disease, chronic inflammation, medications, and other medical conditions can cause many of the same symptoms. That is why I believe in investigating instead of guessing.
How Does This Affect Weight Loss?
This is the part that often gets oversimplified.
Thyroid dysfunction does not eliminate the importance of a calorie deficit. It can change the physiology that determines how difficult creating and sustaining that deficit becomes.
Lower thyroid hormone activity can affect resting energy expenditure, body temperature, muscle function, recovery, energy levels, glucose metabolism, and lipid metabolism.
Now think about the person who is exhausted all day, cold, struggling to exercise, recovering poorly, and losing muscle.
Then tell that person weight loss is simply a matter of trying harder.
That is where the conversation loses me.
Thyroid Hormone Is Not a Weight Loss Drug
None of this means everyone struggling to lose weight should take thyroid hormone.
Too much thyroid hormone can cause palpitations, heart rhythm abnormalities, anxiety, insomnia, muscle loss, and bone loss. That is not optimization. That is hyperthyroidism.
The goal is to determine whether thyroid physiology could be contributing to someone’s symptoms, evaluate the whole person, and treat appropriately when treatment is indicated.
There is a massive difference between optimizing thyroid hormone because a patient needs it and abusing thyroid hormone as a weight loss medication.
Final Thoughts
If you are eating well, prioritizing protein, exercising consistently, and maintaining a calorie deficit but the scale barely moves, the answer probably is not to simply keep cutting calories until you are miserable.
Sometimes we need to ask a better question:
Why has weight loss become so difficult?
For some people, insulin resistance is part of the answer. For others, thyroid dysfunction may be contributing. And for many people, several metabolic and hormonal problems are happening at the same time.
Being told your TSH is “normal” should not automatically end the investigation when you are still experiencing symptoms. TSH has a role in the conventional diagnosis of primary hypothyroidism and hyperthyroidism, but it is not a complete measurement of thyroid hormone physiology throughout the body.
Symptoms matter. T3 matters. Metabolic health matters.
The goal is not simply to make a laboratory value look normal. It is to figure out why you do not feel well and what your body may be trying to tell us.
Next in the Series
In Part 3, we look at another frequently overlooked contributor to stubborn weight gain and weight loss resistance: chronic inflammation.