If you are tired, gaining weight, losing hair, feeling cold, or struggling with brain fog, there is a good chance you have asked your doctor to check your thyroid. And there is an equally good chance your doctor ordered a TSH only, told you it was "normal," and sent you home.
TSH is a useful screening test. It is not a thyroid evaluation. Here is what you should actually be testing and how to interpret it.
Why TSH alone misses the picture
TSH (thyroid-stimulating hormone) is produced by the pituitary gland and tells the thyroid to make hormones. When thyroid hormones drop, TSH rises. When thyroid hormones are adequate, TSH stays low.
The problem: TSH is a lagging indicator. It is the last domino to fall. In early thyroid dysfunction — particularly autoimmune thyroiditis (Hashimoto's) — the thyroid can be under attack for years while TSH stays within the reference range because the gland is compensating by working harder.
By the time TSH goes clearly out of range, you may have been symptomatic for months or years.
The complete thyroid panel
| Test | What it measures | Why it matters |
|---|---|---|
| TSH | Pituitary signal to the thyroid | Screening test — but normal doesn't rule out dysfunction |
| Free T4 | Unbound thyroxine (the storage hormone) | Shows thyroid production capacity |
| Free T3 | Unbound triiodothyronine (the active hormone) | The hormone that actually drives metabolism |
| Reverse T3 | Inactive T3 metabolite | Elevated in chronic stress, illness, caloric restriction — blocks T3 |
| TPO antibodies | Immune attack on thyroid peroxidase enzyme | Positive in 90%+ of Hashimoto's cases |
| TG antibodies | Immune attack on thyroglobulin | Additional autoimmune marker — catches the 5-10% TPO misses |
The full panel costs $80-150 through direct-to-consumer labs. Most doctors will order TSH and free T4 if you ask. Getting free T3, reverse T3, and antibodies may require you to be more direct — or order them yourself.
How to read your results
Scenario 1: TSH high, free T4 low — overt hypothyroidism
This is the textbook pattern. Your thyroid is underproducing and the pituitary is screaming at it to work harder. Your doctor will likely start levothyroxine. This one rarely gets missed.
Scenario 2: TSH normal-high, free T4 normal, TPO antibodies positive — early Hashimoto's
This is the pattern that gets dismissed. TSH is 3.2 (within range), free T4 is fine, but your TPO antibodies are 250+ IU/mL. Your immune system is attacking your thyroid, but the gland is compensating — for now. Symptoms are present but your labs look "normal."
This is where many patients are told "your thyroid is fine, it must be something else." But the antibody elevation means your thyroid is on a trajectory toward failure. Monitoring, lifestyle intervention, and sometimes early treatment are warranted.
Scenario 3: TSH normal, free T4 normal, free T3 low — conversion problem
Your thyroid produces T4 (the storage form), which must be converted to T3 (the active form) in peripheral tissues, primarily the liver and kidneys. Some people have impaired conversion due to:
- Nutrient deficiencies (selenium, zinc, iron)
- Chronic stress (cortisol inhibits conversion)
- Caloric restriction
- Liver dysfunction
- Chronic inflammation
Their TSH and T4 look fine, but free T3 is in the basement — and they feel hypothyroid. This pattern is invisible if you only test TSH.
Scenario 4: Normal T4 and T3, elevated reverse T3 — the stress pattern
Reverse T3 (rT3) is an inactive metabolite of T4. Under chronic stress, illness, caloric restriction, or overtraining, the body shunts T4 toward rT3 instead of active T3. It is a protective mechanism — your body is downregulating metabolism because it perceives a threat.
A high reverse T3 with a normal or low-normal free T3 explains symptoms of hypothyroidism in people whose standard labs look fine. The fix is usually addressing the underlying stressor, not thyroid medication.
The TSH reference range debate
The standard reference range for TSH is 0.4-4.0 mIU/L (some labs use 0.5-4.5). This range was derived from population data that included people with undiagnosed thyroid disease.
Many endocrinologists and functional medicine practitioners argue that the optimal range is narrower:
| TSH range | Interpretation |
|---|---|
| 0.5-1.5 mIU/L | Optimal for most people |
| 1.5-2.5 mIU/L | Acceptable but worth watching if symptomatic |
| 2.5-4.0 mIU/L | Suboptimal — investigate further, especially with antibodies |
| Above 4.0 mIU/L | Clearly elevated — warrants treatment discussion |
| Below 0.4 mIU/L | May indicate hyperthyroidism — needs workup |
A TSH of 3.8 is "in range" but is not the same as a TSH of 1.2. Context matters.
Nutrients that support thyroid function
- Selenium — 200 mcg/day has been shown to reduce TPO antibodies in Hashimoto's patients. Brazil nuts are the richest food source.
- Iodine — essential for thyroid hormone synthesis, but excess can worsen autoimmune thyroiditis. Get enough (150 mcg/day) but don't megadose.
- Zinc — required for T4-to-T3 conversion.
- Iron — thyroid peroxidase (the enzyme that makes thyroid hormones) requires iron as a cofactor. Iron deficiency and hypothyroidism frequently coexist.
- Vitamin D — low vitamin D is associated with higher rates of thyroid autoimmunity.
How Merios helps
Upload your thyroid panel to Merios and track TSH, free T4, free T3, and antibodies over time. See them alongside your resting heart rate and HRV from Apple Watch — because thyroid function directly affects both. When you start a new intervention (selenium, levothyroxine dose change, stress reduction protocol), you will see whether the trend is moving in the right direction at your next blood draw.
Track your thyroid panel with Merios →
This article is for informational purposes only. Thyroid conditions require proper medical evaluation and management. Consult an endocrinologist or experienced healthcare provider.