Thyroid Tests — TSH, Free T4 and Free T3
The thyroid is a small, butterfly-shaped gland in the neck that produces hormones affecting nearly every organ in the body.
Thyroid hormones help regulate metabolism, body temperature, heart rate, energy levels, and many other functions.
Thyroid problems become more common with age, particularly in women. But symptoms such as fatigue, constipation, feeling cold, and difficulty concentrating can easily be mistaken for normal aging.
Three blood tests—TSH, Free T4, and Free T3—help doctors evaluate thyroid function. Understanding the differences can make laboratory results easier to interpret.
TSH — Thyroid-Stimulating Hormone
TSH is produced by the pituitary gland in the brain, not by the thyroid itself.
Its job is to signal the thyroid to produce more hormone when the body needs it.
When thyroid hormone levels fall, TSH usually rises. When thyroid hormone levels are excessive, TSH usually falls.
This feedback system makes TSH a useful initial screening test for most people.
A commonly used reference range is approximately 0.4–4.0 mIU/L, although laboratory ranges vary.
A high TSH may indicate an underactive thyroid (hypothyroidism), while a low TSH may indicate an overactive thyroid (hyperthyroidism).
However, TSH results must be interpreted in context.
Free T4 — The Main Thyroid Hormone
Thyroxine (T4) is the primary hormone released by the thyroid gland.
Most T4 circulates attached to proteins. Free T4 measures the small fraction that is not protein-bound and is available to tissues.
The body converts T4 into the more biologically active hormone T3.
Free T4 is particularly helpful when TSH is abnormal.
For example:
High TSH with low Free T4: Usually indicates overt primary hypothyroidism.
High TSH with normal Free T4: Often indicates subclinical hypothyroidism.
Low TSH with high Free T4: May indicate hyperthyroidism.
Low or normal TSH with low Free T4: May suggest a pituitary or hypothalamic problem.
These patterns are useful starting points, but diagnosis also depends on symptoms, medications, and other findings.
Free T3 — The Active Thyroid Hormone
Triiodothyronine (T3) is the more biologically active thyroid hormone.
Much of the body's T3 is produced by converting T4 into T3 in tissues.
Free T3 measures the unbound portion circulating in the blood.
Some functional-medicine practitioners emphasize Free T3 because it is the active hormone and because conversion from T4 to T3 may vary.
However, Free T3 testing is not routinely recommended for diagnosing hypothyroidism. Results can be affected by illness, and some Free T3 laboratory methods have limitations.
T3 testing is often more useful when evaluating suspected hyperthyroidism.
A normal Free T3 level does not necessarily exclude thyroid disease, and a low level does not automatically mean that T3 treatment is needed.
What Is Hashimoto's Thyroiditis?
Hashimoto's thyroiditis is an autoimmune condition in which the immune system attacks the thyroid gland.
Over time, this can reduce thyroid hormone production.
It is one of the most common causes of hypothyroidism in the United States.
Doctors may order thyroid antibody tests, especially:
TPO antibodies: Antibodies against thyroid peroxidase
Thyroglobulin antibodies: Antibodies against thyroglobulin
Positive antibodies can support a diagnosis of autoimmune thyroid disease.
However, some people have positive thyroid antibodies while their thyroid hormone levels remain normal.
A positive antibody test does not automatically mean thyroid medication is needed.
What Happens to Thyroid Tests as We Age?
TSH levels tend to increase somewhat with age, even in otherwise healthy people.
This means that a mildly elevated TSH in someone in their 80s may not carry the same significance as an identical result in a younger adult.
Research has also found that treating mild subclinical hypothyroidism in many older adults does not substantially improve fatigue, general symptoms, or quality of life.
Treatment decisions therefore depend on several factors, including:
How high the TSH is
Whether Free T4 is low
Symptoms
Cardiovascular health
Other medical conditions
Whether thyroid antibodies are present
Avoiding overtreatment is particularly important in older adults.
Excess thyroid hormone can increase the risk of atrial fibrillation, bone loss, and fractures.
When Is Thyroid Treatment Appropriate?
For overt hypothyroidism, where TSH is elevated and Free T4 is low, treatment with thyroid hormone is generally recommended.
The standard initial treatment is levothyroxine, a synthetic form of T4.
For subclinical hypothyroidism, where TSH is mildly elevated but Free T4 remains normal, treatment is more individualized.
In older adults, especially those over 70, a modestly elevated TSH often does not require immediate medication.
Persistent TSH elevations above approximately 10 mIU/L generally prompt stronger consideration of treatment, although age and overall health still matter.
What About Armour Thyroid and T3 Treatment?
Armour Thyroid is a prescription medication made from desiccated animal thyroid glands. It contains both T4 and T3, unlike levothyroxine, which provides only T4.
Armour Thyroid was widely used before synthetic thyroid hormones became available and continues to be prescribed by some physicians.
The American Thyroid Association recommends levothyroxine as the standard initial treatment for hypothyroidism. However, it also recognizes that desiccated thyroid extract may be the preferred treatment for some patients.
Clinical trials have generally not found that Armour Thyroid produces better results for everyone. Nevertheless, some patients report feeling better on Armour or other treatments containing both T4 and T3.
In a 2013 randomized trial, nearly half the participants preferred desiccated thyroid extract over levothyroxine, although overall quality-of-life improvements were not significantly different.
A 2021 trial also found no overall superiority, but patients who remained most symptomatic on levothyroxine showed greater preference for T3-containing treatments.
Research reviews suggest that patients may prefer combination therapies, although patient preference does not necessarily establish better long-term health outcomes.
Because Armour contains T3, careful dosing and monitoring are important, particularly for older adults. Excess thyroid hormone can increase the risk of atrial fibrillation and bone loss.
The best treatment may depend on the individual patient's response, laboratory results, medical history, and preferences.
Why Do Functional-Medicine Practitioners Sometimes Order More Tests?
Some functional-medicine practitioners prefer a broader thyroid panel, including TSH, Free T4, Free T3, and thyroid antibodies.
Their reasoning is that symptoms may persist even when TSH falls within the laboratory reference range.
They may also investigate nutritional deficiencies, autoimmune disease, stress, and other metabolic factors.
Some of these evaluations can be useful, particularly when symptoms remain unexplained.
However, broader testing does not necessarily lead to better outcomes, and there is insufficient evidence that routinely adjusting thyroid medication to achieve higher Free T3 levels improves health.
Can Diet or Supplements Improve Thyroid Function?
Adequate iodine and selenium are necessary for normal thyroid hormone production.
However, taking more than the body needs does not necessarily improve thyroid function.
Excess iodine can actually worsen certain thyroid disorders, including autoimmune thyroid disease.
Selenium supplementation has been studied in Hashimoto's thyroiditis. Some trials show reductions in thyroid antibody levels, but evidence that this translates into meaningful improvements in symptoms or long-term thyroid function remains uncertain.
People with thyroid disease should be cautious about supplements marketed as thyroid boosters, particularly products containing iodine or thyroid hormones.
The Bottom Line
TSH is usually the most useful starting point for evaluating thyroid function, with Free T4 providing important additional information when needed.
Free T3 and thyroid antibody tests can be helpful in selected circumstances, but more testing is not always better.
For older adults, mild TSH elevations may reflect age-related changes rather than thyroid disease requiring treatment.
Levothyroxine remains the standard initial treatment, but some patients prefer or respond better to Armour Thyroid or other T3-containing therapies.
The goal is not simply to achieve an ideal laboratory number. It is to identify genuine thyroid problems, select appropriate treatment, and avoid both undertreatment and overtreatment.
Learn More
Explains how TSH, Free T4, and T3 are used to evaluate thyroid function and why different tests are appropriate in different situations.
A randomized trial involving 70 patients found no significant overall improvement in quality of life with desiccated thyroid extract. However, nearly half the participants preferred it to levothyroxine.
This randomized trial compared levothyroxine, desiccated thyroid extract, and combined T4/T3 therapy. Overall results were similar, but patients with the most persistent symptoms on levothyroxine tended to prefer and respond better to T3-containing treatments.
The association continues to recommend levothyroxine as the standard initial treatment but acknowledges that desiccated thyroid extract is the treatment of choice for some patients and supports individualized treatment.