Your lab report shows three values: TSH, free T4, and free T3. What does "elevated" actually mean? When does treatment matter — and when is it just monitoring?
This guide explains the pituitary–thyroid feedback loop, the ATA reference ranges, and the eight clinical patterns you'll see on a thyroid panel.
How the thyroid is regulated
The thyroid produces T4 (thyroxine, ~80 %) and T3 (triiodothyronine, ~20 %). T3 is the biologically active form — it acts on cells and controls metabolism, heart rate, and body temperature. T4 is the storage form, converted to T3 in tissues as needed.
The pituitary regulates output via TSH (thyroid-stimulating hormone) in a negative feedback loop: when thyroid hormones drop, the pituitary releases more TSH; when they rise, TSH is suppressed.
The diagnostic principle that follows: TSH moves opposite to thyroid function. High TSH points to underactivity, low TSH to overactivity.
Reference ranges
| Marker | Reference range | Alternative units |
|---|---|---|
| TSH | 0.4 – 4.0 mIU/L | = µIU/mL |
| Free T4 | 0.8 – 1.8 ng/dL | 10.3 – 23.2 pmol/L |
| Free T3 | 2.3 – 4.2 pg/mL | 3.5 – 6.5 pmol/L |
Source: American Thyroid Association (ATA). Pregnancy, age, and lab method can shift the ranges.
The eight clinical patterns
| TSH | fT4 | Pattern |
|---|---|---|
| Normal | Normal | Euthyroid |
| High | Low | Primary hypothyroidism |
| High | Normal | Subclinical hypothyroidism |
| Low | High | Primary hyperthyroidism |
| Low | Normal | Subclinical hyperthyroidism |
Hypothyroidism — when output is too low
Most common cause: Hashimoto's thyroiditis, an autoimmune attack on the thyroid that gradually destroys tissue. T4 falls, TSH climbs. Symptoms: fatigue, weight gain, cold intolerance, constipation, dry skin, low mood.
Treatment: levothyroxine, taken on an empty stomach. Target TSH 0.5–2.5 mIU/L. Subclinical disease (TSH high, T4 normal) is often watched until TSH exceeds 10 mIU/L or symptoms appear.
Hyperthyroidism — when output is too high
Main causes: Graves' disease (autoimmune, TRAb antibodies), autonomous nodules, thyroiditis. Symptoms: palpitations, weight loss with normal appetite, heat intolerance, anxiety, insomnia, tremor.
Treatment depends on cause: anti-thyroid drugs (methimazole), radioactive iodine, or surgery. Subclinical hyperthyroidism needs monitoring because of atrial fibrillation and bone density risks.
Interpret your thyroid panel
Enter TSH, free T3 and free T4 — instant pattern matching against ATA reference ranges.
Calculate for free →Related calculators
Thyroid hormones drive your basal metabolic rate directly — hypothyroidism can cut it by up to 15 %. Your total daily energy expenditure follows. If metabolic markers are off, also check your diabetes risk, since Hashimoto's and type 1 diabetes commonly cluster together.
Bottom line
TSH is the most sensitive marker; free T4 and free T3 fill in the picture when something looks off. Subclinical patterns are common and often need only follow-up. Use our Thyroid Function Calculator as orientation — not as a substitute for clinical evaluation.
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