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Thyroid Function Calculator

Map TSH, free T3 and free T4 to a thyroid function pattern using ATA adult reference ranges.

Reference range: 0.4 – 4.0 mIU/L

Reference range: 0.8 – 1.8 ng/dL

Reference range: 2.3 – 4.2 pg/mL

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Reference ranges (adults, ATA guidelines)

MarkerReference rangeMeaning
TSH0.4 – 4.0 mIU/LPituitary signal to the thyroid
Free T40.8 – 1.8 ng/dL (10.3 – 23.2 pmol/L)Main storage hormone (precursor)
Free T32.3 – 4.2 pg/mL (3.5 – 6.5 pmol/L)Active form, acts on cells

How it works

The calculator maps TSH, free T4 and free T3 against ATA reference ranges and matches the pattern to one of eight categories. The relationship between the three values distinguishes primary from subclinical forms of hyper- and hypothyroidism. The pituitary releases TSH to drive thyroid hormone production — when the gland under- or over-produces, the values shift in characteristic ways.

Note: This is an educational tool, not a diagnosis. A clinical assessment requires history, physical exam, and often antibodies (TPO, TRAb) plus ultrasound.

Frequently Asked Questions

What does TSH measure?+
TSH (thyroid-stimulating hormone) is released by the pituitary gland and tells the thyroid to make T4 and T3. High TSH suggests an underactive thyroid (the pituitary is 'shouting louder'); low TSH suggests an overactive thyroid. TSH is the most sensitive single marker.
What's the difference between T3 and T4?+
T4 (thyroxine) is the main storage form — about 80 % of thyroid output. T3 (triiodothyronine) is the biologically active form, converted from T4 in tissues. 'Free' (fT4, fT3) refers to the unbound, active fraction.
What is subclinical thyroid disease?+
Subclinical hypothyroidism: TSH elevated, free T4 still normal. Subclinical hyperthyroidism: TSH suppressed, free T4 and T3 normal. Symptoms may be subtle or absent.
What symptoms suggest a thyroid problem?+
Hypothyroidism: fatigue, weight gain, cold intolerance, constipation, dry skin. Hyperthyroidism: palpitations, weight loss, heat intolerance, anxiety, insomnia. Hashimoto's and Graves' are the leading autoimmune causes.
How often should thyroid values be checked?+
Normal function: every 1–2 years. On levothyroxine: every 6–12 months. Subclinical findings: recheck in 4–8 weeks. In pregnancy with hypothyroidism: every 4–6 weeks.
Do medications affect thyroid values?+
Yes. High-dose biotin, lithium, amiodarone, estrogen, and glucocorticoids can shift the values. Hold biotin for 48 hours before testing.
What is Hashimoto's thyroiditis?+
The most common cause of hypothyroidism — an autoimmune disease where the immune system attacks the thyroid. Diagnosed via TPO antibodies. Treated with levothyroxine once TSH is persistently elevated.

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Thyroid Function Calculator: Read TSH, T3 and T4 Like a Pro

9 min