How Thyroid Hormones Actually Work

The thyroid produces two primary hormones: T4 (thyroxine) and T3 (triiodothyronine). T4 is the storage form. It's produced in the largest quantity but is largely inactive until it's converted into T3. T3 is the active form that the body's cells can actually use, and it is the hormone responsible for most of what we associate with thyroid function: metabolic rate, body temperature regulation, energy production, cognitive function, and more.

According to NIH resources on thyroid hormone synthesis, approximately 80 percent of T3 in the body comes from peripheral conversion of T4, meaning the thyroid itself doesn't produce most of the active hormone. The conversion happens in tissues throughout the body, particularly the liver and kidneys. When that conversion process is impaired, a person can have normal TSH and normal T4 but still have inadequate T3 at the cellular level, which produces all the symptoms of hypothyroidism while registering as "normal" on a standard panel.

TSH reflects how hard the pituitary is working to stimulate the thyroid. It can stay within range while other parts of the system are underperforming.

What Standard Testing Misses

T4 to T3 conversion problems. Low free T3 relative to free T4 indicates that conversion is impaired. Factors that impair conversion include chronic stress and elevated cortisol, nutrient deficiencies (particularly selenium and zinc), gut dysbiosis and inflammation, certain medications, and liver dysfunction. None of these show up on a TSH-only panel. A patient with impaired conversion may have normal TSH, normal T4, and persistently low free T3, with full hypothyroid symptoms and no diagnosis to explain them.

Hashimoto's thyroiditis. Hashimoto's is the most common cause of hypothyroidism in women, and it is an autoimmune condition, meaning the immune system is attacking thyroid tissue. Thyroid antibody testing (anti-TPO and anti-thyroglobulin antibodies) is required to identify it. A patient can have active Hashimoto's with normal TSH and T4 for years before the antibody load damages the thyroid enough to shift those markers. Without antibody testing, the diagnosis is missed entirely, and the patient is told her thyroid is fine while her immune system continues to attack it.

Subclinical hypothyroidism. TSH values between 2.5 and 4.5 mIU/L fall within the standard laboratory reference range but are considered subclinical or borderline by many functional medicine clinicians. Research in Frontiers in Endocrinology has examined the association between subclinical hypothyroidism and symptomatic presentations, including fatigue, cognitive changes, and metabolic effects, particularly in women. Standard practice often does not treat at this range. In clinical practice at Soal Wellness, we evaluate symptom burden alongside the numbers rather than treating the reference range as a definitive answer.

What a Complete Thyroid Evaluation Looks Like

At a minimum, a clinically useful thyroid panel includes:

- TSH, the standard screening marker

- Free T4, available T4 before conversion

- Free T3, the active hormone; the most clinically relevant marker for how the patient is actually functioning

- TPO antibodies (anti-thyroid peroxidase), primary autoimmune marker for Hashimoto's

- Thyroglobulin antibodies, secondary autoimmune marker, adds sensitivity

When clinically indicated, the panel may also include additional markers to assess the nutrient status that supports thyroid function.

Results are read in context, not in isolation. A free T3 in the lower third of the reference range in a patient with significant fatigue, hair loss, and weight changes is a different clinical picture than the same number in a patient who feels well. Normal and optimal are not the same thing, and reference ranges are population averages, not individual targets.

The Thyroid-Hormone Connection in Perimenopause

Thyroid dysfunction and perimenopause share an overlapping symptom profile that creates one of the most common diagnostic delays in women's health. Fatigue, brain fog, weight changes, mood shifts, hair thinning, irregular periods, and sleep disruption are symptoms of both. Because perimenopause is often the first thing considered in women in their 40s and early 50s, thyroid dysfunction frequently goes untested or gets blamed on the hormonal transition rather than investigated on its own terms.

The two conditions also interact. Conversely, low thyroid function can suppress sex hormone production and affect the metabolism of estrogen, creating a cycle where each system's dysfunction reinforces the other.

At Soal Wellness, thyroid markers are a standard part of any hormone evaluation, not a separate workup only pursued when symptoms are dramatic. The goal is to see where both systems are before attributing symptoms to one cause or the other.

Treatment Framing: Natural Thyroid Support

When thyroid function is suboptimal, whether subclinical hypothyroidism, confirmed low free T3, or Hashimoto's, treatment is individualized and builds on the clinical picture.

Foundational support always comes first: addressing nutrient deficiencies that impair thyroid function (selenium, zinc, iodine, vitamin D), managing chronic stress that suppresses T3 conversion, and evaluating gut health, since gut inflammation and dysbiosis affect both hormone absorption and conversion.

When natural thyroid support is clinically warranted, options include natural desiccated thyroid (NDT), which contains both T4 and T3, or individualized T4/T3 combination therapy. Standard T4-only treatment (levothyroxine) may be appropriate for some patients, but for those with conversion problems, T4 alone does not address the underlying issue. Dosing is reassessed four to eight weeks after initiation based on labs and symptom response, not on TSH alone.

For Hashimoto's, the focus extends beyond the thyroid itself to include immune regulation, inflammation management, and identifying triggers, including dietary factors and environmental contributors, that drive antibody activity. Managing the autoimmune component can slow disease progression meaningfully, even when thyroid hormone levels are currently stable.

When to Pursue a Full Thyroid Evaluation

If you have persistent fatigue, brain fog, hair thinning, unexplained weight changes, or cold intolerance, and have been told your thyroid is normal based on TSH alone, a complete panel is worth requesting. The American Thyroid Association recommends TSH as a screening tool, but clinical guidelines also support broader testing when symptoms persist despite normal TSH.

If you are in your 40s or early 50s and experiencing any of these symptoms alongside signs of perimenopause, both pictures deserve evaluation, separately and together.