Understanding Thyroid Values: TSH, Free T3, Free T4

TL;DR: TSH alone is not enough. Reference range 0.4 to 4.0 mIU/L, optimal 1.0 to 2.0. fT4 (reference 0.9 to 1.7 ng/dL, optimal 1.2 to 1.5) and fT3 (reference 2.0 to 4.4 pg/mL, optimal 3.0 to 4.0) show how much active hormone actually reaches your cells. Hashimoto affects 5 to 10% of the population, with women 5 times more likely to be affected. Always test TSH plus fT3 plus fT4 — and add TPO antibodies if autoimmune disease is suspected.

This article does not replace medical advice — consult a doctor for thyroid issues or abnormal results.

What Your Thyroid Does — in 60 Seconds

Your thyroid sits at the front of your neck and produces two hormones: T4 (thyroxine) and T3 (triiodothyronine). T4 is the storage hormone — it circulates in your blood until it is needed. T3 is the active hormone, 3 to 5 times more potent than T4. About 80% of T3 is produced by converting T4 in the liver and kidneys.

The control center sits in your brain. The pituitary gland measures T3/T4 levels and sends TSH (thyroid-stimulating hormone) as a signal to the thyroid. Too little T3/T4 in the blood? TSH rises to push the thyroid harder. Too much? TSH drops.

Why this matters: If this feedback loop stalls at any point — thyroid producing too little, conversion failing, pituitary responding slowly — a single marker will not show it. You need the interplay of TSH, fT3, and fT4. For a broader overview of key blood markers, read the blood values guide.

TSH: The Marker Everyone Knows — and Why It Is Not Enough

TSH is the most commonly tested thyroid marker. The reference range is 0.4 to 4.0 mIU/L. For optimal health, many endocrinologists recommend a tighter range: 1.0 to 2.0 mIU/L.

The problem: A normal TSH does not mean your thyroid is functioning well. TSH responds with a delay of 6 to 8 weeks. Your fT3 may have been low for weeks while TSH still sits at 2.5 mIU/L and your doctor says everything is fine.

A concrete example: Your patient reports fatigue and weight gain. TSH reads 3.2 mIU/L — within the reference range. But fT3 comes back at 2.3 pg/mL, below the optimal range. Only the full panel reveals the conversion problem.

Additional pitfalls: TSH follows a circadian rhythm. Peak levels occur between 2 and 4 AM, and by afternoon TSH can drop by 50%. Always test between 7 and 9 AM, fasting, and at the same time of day. Trend comparisons only work with consistent timing.

fT3 and fT4: The Actual Thyroid Hormones

While TSH is just the control signal, fT3 and fT4 measure the hormones themselves. The “f” stands for free — the fraction not bound to transport proteins and biologically active.

Marker	Reference Range	Optimal	What It Shows
fT4 (free thyroxine)	0.9–1.7 ng/dL	1.2–1.5 ng/dL	Thyroid production
fT3 (free triiodothyronine)	2.0–4.4 pg/mL	3.0–4.0 pg/mL	Active hormone, conversion

fT4 shows whether the thyroid produces enough hormone. Low fT4 with high TSH confirms hypothyroidism. fT4 has a half-life of 7 days and fluctuates little — a stable marker.

fT3 shows whether T4-to-T3 conversion is working. fT3 has a half-life of only 24 hours and responds faster to changes. Chronic stress, calorie restriction, and sleep deprivation often lower fT3 first.

The fT3/fT4 ratio is your conversion indicator. Divide fT3 (in pg/mL) by fT4 (in ng/dL). A result above 0.27 suggests good conversion. Below 0.20 indicates a likely conversion problem — check selenium, iron, and stress levels.

The Complete Thyroid Panel

A single TSH test costs about 15 USD and delivers 30% of the information. The complete panel costs 80 to 130 USD and gives you the full picture. Here is the breakdown:

Minimum panel (3 markers):

• TSH — pituitary control signal
• fT3 — active thyroid hormone
• fT4 — storage hormone, conversion starting point

Autoimmune screening (2 additional markers):

• TPO antibodies (anti-TPO) — positive above 34 IU/mL, primary marker for Hashimoto
• Thyroglobulin antibodies (Tg-Ab) — catches the 10% of Hashimoto cases that are TPO-negative

Extended diagnostics (when indicated):

• rT3 (reverse T3) — produced during stress, inflammation, and calorie restriction instead of T3. Useful when fT3 is low with normal fT4
• Selenium in whole blood — conversion co-factor
• Ferritin — thyroid function co-factor (see the ferritin guide)
• Vitamin D (25-OH) — correlates with TPO antibody levels (see the vitamin D guide)

For proper blood draw preparation, read the biomarker baseline checklist.

Hashimoto: The Most Common Thyroid Disease

Hashimoto thyroiditis is an autoimmune condition where the immune system attacks and slowly destroys the thyroid. It affects 5 to 10% of the population. Women are 5 times more likely to develop it, especially between ages 30 and 50.

Diagnosis rests on two pillars: elevated TPO antibodies above 34 IU/mL and a typical echo pattern on ultrasound. About 90% of Hashimoto patients have elevated TPO antibodies. The remaining 10% show only elevated Tg antibodies or are seronegative — ultrasound helps in those cases.

Symptoms are tricky because they overlap with many other causes:

• Fatigue and lack of motivation (similar to iron deficiency)
• Weight gain despite normal eating habits
• Hair loss and dry skin
• Cold sensitivity
• Depressed mood (similar to vitamin D deficiency)
• Menstrual irregularities in women

A concrete example: Your colleague has been tired with gradual weight gain for a year. Her doctor tests TSH (2.8 mIU/L — normal) and sends her home. On her own initiative, she gets TPO antibodies tested: 280 IU/mL. The Hashimoto diagnosis explains everything — even though TSH was still in the normal range.

Tracking Hashimoto: Test TSH, fT3, fT4, and TPO antibodies every quarter. TPO antibody trends show whether autoimmune activity is increasing or calming down. Track these systematically using long-term biomarker tracking.

Subclinical Hypothyroidism: The Gray Zone

Subclinical hypothyroidism means TSH is between 4.0 and 10 mIU/L while fT3 and fT4 remain in the normal range. You have no obvious symptoms — or only mild ones like slight fatigue and dry skin.

The numbers: 5 to 10% of the population is affected. About 20% progress to overt hypothyroidism within 10 years. The risk increases with higher TSH levels and positive TPO antibodies.

When to treat? Guidelines vary, but clear decision points exist:

• TSH above 10 mIU/L — L-thyroxine therapy recommended, even without symptoms
• TSH 7 to 10 mIU/L with symptoms — trial therapy is reasonable
• TSH 4 to 7 mIU/L without symptoms — observe, recheck every 3 to 6 months
• TPO antibodies positive — lower threshold for therapy, because progression risk is higher

A concrete example: Your TSH is 5.8 mIU/L, fT3 is 2.9 pg/mL, and fT4 is 1.1 ng/dL. Clinically, you are in the gray zone. If your TPO antibodies are 180 IU/mL, progression risk is high and a therapy trial makes sense. If TPO antibodies are negative and you are symptom-free, close monitoring is sufficient.

Co-Factors: Why Selenium, Iron, and Vitamin D Matter

Thyroid hormones do not form in isolation. Three micronutrients are directly involved in production and conversion. If one is missing, even the best L-thyroxine dose cannot work properly.

Selenium. The thyroid contains more selenium per gram of tissue than any other organ. Selenium activates the deiodinase enzymes that convert T4 into T3. Studies show that 200 micrograms of selenomethionine per day reduces TPO antibodies in Hashimoto patients by 20 to 40%. Without adequate selenium (target: 100 to 130 micrograms per liter in whole blood), conversion stalls.

Iron. Iron is a co-factor for thyroid peroxidase, the enzyme that synthesizes thyroid hormones. Ferritin below 30 ng/mL measurably impairs thyroid function. Women with iron deficiency are twice as likely to develop thyroid problems. Before increasing your L-thyroxine dose, check your ferritin. The complete guide on iron status and supplementation is in the ferritin and iron deficiency guide.

Vitamin D. A 25-OH vitamin D level above 50 ng/mL correlates with lower TPO antibodies. Studies show an inverse relationship: the higher your vitamin D, the lower the autoimmune activity. Target: 50 to 70 ng/mL. Details on proper dosing in the vitamin D guide.

For a solid introduction to supplement basics, check the supplement beginners guide.

Preparing for Your Blood Test

Incorrect preparation distorts thyroid values more than most other markers. Follow these five rules:

1. Skip L-thyroxine. If you take L-thyroxine, take it after the blood draw. Your last dose should be at least 24 hours ago. Otherwise, fT4 can be artificially elevated by up to 30%.

2. Pause biotin for 72 hours. Biotin interferes with immunoassays and can make TSH appear falsely low and fT3/fT4 falsely high. Many multivitamins and hair-skin-nails supplements contain 2,500 to 10,000 micrograms of biotin — check the label.

3. Test in the morning, fasting. TSH follows a circadian rhythm, peaking between 2 and 4 AM. Between 7 and 9 AM, TSH is still high enough for a meaningful reading. By afternoon, TSH can drop by 50%.

4. Always test at the same time of day. Trend comparisons only work with consistent timing. Record the time with every measurement.

5. Document context. Stress, sleep, infections, and menstrual cycle phase affect thyroid values. Without context, later comparisons become unreliable. The complete preparation checklist is in the baseline guide.

Tracking: How Often and What to Monitor

A single test is a snapshot. Regular tracking reveals trends — and trends are what drive therapy adjustments. Here is the schedule that works in practice.

Initial diagnosis: Test TSH, fT3, and fT4 every 6 to 8 weeks until values stabilize. With L-thyroxine adjustment, this typically takes 3 to 4 cycles.

Stable phase: 2 checks per year are sufficient. Ideally once in spring and once in fall, because thyroid values show seasonal variation.

With Hashimoto: Quarterly TSH, fT3, fT4, plus TPO antibodies. The TPO antibody trend shows whether autoimmune activity is increasing or calming down under selenium, vitamin D, and dietary adjustments.

Set target values. Enter specific targets into your tracking — for example TSH 1.5 mIU/L, fT3 3.5 pg/mL. Without targets, you are optimizing blindly. In Lab2go, you can see at a glance how close your current value is to your benchmark.

Document context. Record the time, L-thyroxine dose, supplement intake, stress level, and cycle phase with every measurement. For a systematic approach to 12-month biomarker tracking, read the long-term tracking guide.

Thyroid Health and Women: Why Extra Attention Is Needed

Thyroid disorders affect women 5 to 8 times more often than men. Hashimoto, subclinical hypothyroidism, and conversion problems are so common among women between 25 and 55 that annual screening is justified.

Menstrual irregularities are often the first sign. Subclinical hypothyroidism can lengthen cycles, cause heavier bleeding, and impair fertility. TSH above 2.5 mIU/L during the fertility planning phase is considered treatable — a tighter threshold than usual.

Pregnancy increases thyroid hormone demand by 30 to 50%. L-thyroxine dosage often needs adjustment as early as the first trimester. TSH target during pregnancy: below 2.5 mIU/L.

Perimenopause overlaps heavily with thyroid symptoms: fatigue, weight gain, mood swings. Before attributing everything to menopause, get a complete thyroid panel.

Thyroid Values at a Glance: Reference Table

Marker	Reference Range	Optimal	When It Is Abnormal
TSH	0.4–4.0 mIU/L	1.0–2.0 mIU/L	Above 4.0: hypothyroidism, below 0.4: hyperthyroidism
fT4	0.9–1.7 ng/dL	1.2–1.5 ng/dL	Below 0.9: production too low
fT3	2.0–4.4 pg/mL	3.0–4.0 pg/mL	Below 3.0: check conversion
TPO-Ab	below 34 IU/mL	below 34 IU/mL	Above 34: Hashimoto suspected
Tg-Ab	below 115 IU/mL	below 115 IU/mL	Above 115: autoimmune process
rT3	9.2–24.1 ng/dL	9–14 ng/dL	Above 20: stress conversion
Selenium (whole blood)	80–120 mcg/L	100–130 mcg/L	Below 80: conversion impaired
Ferritin	15–150 ng/mL (F)	60–120 ng/mL	Below 30: thyroid function compromised

Save this table for reference — you will need it every time you review your results.

Conclusion: Test More Than Just TSH

Your thyroid controls your metabolism, energy, and mood. Yet most doctors only test TSH — and miss subclinical problems, conversion issues, and early Hashimoto signs.

Your next step: Order the full panel at your next appointment — TSH, fT3, fT4, plus TPO antibodies. Prepare with the baseline checklist. Document everything digitally and track trends over time. Check out the Lab2go features or compare the plans and pricing to get started.

If your ferritin or vitamin D is also low, your thyroid is missing important co-factors. Check those values first before adjusting your L-thyroxine dose. The individual guides on ferritin and vitamin D show you exactly how.

This article does not replace medical advice. If you suspect a thyroid disorder, have abnormal values, or experience symptoms, always consult a doctor. Self-tracking complements medicine — it does not replace it.