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Thyroid Function Calculator
Estimates thyroid status and interprets TSH, Free T4, and Free T3 levels to classify hypothyroid, euthyroid, or hyperthyroid states.
Calculator
Formula
Thyroid function is assessed by evaluating three key hormone levels: TSH (Thyroid-Stimulating Hormone, reference range 0.4–4.0 mIU/L), Free T4 (FT4, reference range 0.8–1.8 ng/dL), and Free T3 (FT3, reference range 2.3–4.2 pg/mL). Elevated TSH with low FT4 indicates primary hypothyroidism; suppressed TSH with elevated FT4 and/or FT3 indicates hyperthyroidism. Normal TSH with normal FT4 and FT3 indicates euthyroid status.
Source: American Thyroid Association (ATA) Clinical Guidelines, 2012; Jameson JL et al., Harrison's Principles of Internal Medicine, 20th ed.
How it works
The thyroid gland regulates metabolism, energy balance, heart rate, and body temperature through the secretion of thyroxine (T4) and triiodothyronine (T3). These hormones are tightly controlled by a negative feedback loop involving the hypothalamus and pituitary gland. The pituitary secretes Thyroid-Stimulating Hormone (TSH) in response to low circulating thyroid hormone levels; when thyroid hormone rises, TSH is suppressed. Because TSH is exquisitely sensitive to even small changes in thyroid hormone levels, it is the most important single test for assessing thyroid function. However, adding Free T4 and Free T3 measurements provides critical additional information, particularly in distinguishing primary from secondary hypothyroidism and in identifying T3 toxicosis.
This calculator uses three reference ranges established by major endocrinology societies: TSH 0.4–4.0 mIU/L, Free T4 0.8–1.8 ng/dL, and Free T3 2.3–4.2 pg/mL. Primary hypothyroidism presents as elevated TSH (often >10 mIU/L) with low Free T4. Subclinical hypothyroidism shows mildly elevated TSH (4–10 mIU/L) with normal Free T4 and T3. Overt hyperthyroidism shows suppressed TSH (<0.4 mIU/L) with elevated Free T4 and/or Free T3. Subclinical hyperthyroidism presents with suppressed TSH but normal free hormone levels. Discordant patterns may indicate secondary hypothyroidism (pituitary dysfunction), non-thyroidal illness, or assay interference.
This tool is widely applicable in clinical practice during routine screening, follow-up of known thyroid disease, monitoring thyroid replacement therapy (levothyroxine), evaluating cardiovascular symptoms such as palpitations or bradycardia, and assessing unexplained weight changes, fatigue, or mood disorders. Endocrinologists, general practitioners, obstetricians (thyroid disorders are common in pregnancy), and cardiologists all rely on thyroid panel interpretation as a standard component of patient workup.
Worked example
Example 1 — Primary Hypothyroidism:
A 45-year-old woman presents with fatigue, weight gain, cold intolerance, and constipation. Laboratory results: TSH = 18.5 mIU/L, Free T4 = 0.5 ng/dL, Free T3 = 1.9 pg/mL.
Interpretation: TSH is markedly elevated (normal: 0.4–4.0 mIU/L), Free T4 is low (normal: 0.8–1.8 ng/dL), and Free T3 is low (normal: 2.3–4.2 pg/mL). This pattern is consistent with overt primary hypothyroidism, likely autoimmune (Hashimoto's thyroiditis). Treatment with levothyroxine is indicated, targeting TSH within the normal range.
Example 2 — Subclinical Hypothyroidism:
A 38-year-old man with mild fatigue. TSH = 6.2 mIU/L, Free T4 = 1.1 ng/dL, Free T3 = 3.0 pg/mL.
Interpretation: TSH is mildly elevated above the upper limit, but Free T4 and Free T3 remain within normal range. This represents subclinical hypothyroidism. Clinical management depends on symptoms, TSH trajectory, anti-TPO antibody status, and cardiovascular risk.
Example 3 — Hyperthyroidism:
A 30-year-old woman with palpitations, tremor, heat intolerance, and weight loss. TSH = 0.02 mIU/L, Free T4 = 3.4 ng/dL, Free T3 = 8.1 pg/mL.
Interpretation: TSH is profoundly suppressed, Free T4 and Free T3 are both markedly elevated, consistent with overt hyperthyroidism. Further workup including thyroid antibodies (TRAb), radioiodine uptake, or thyroid ultrasound is indicated to distinguish Graves' disease from toxic nodular goitre.
Limitations & notes
This calculator provides an educational classification based on standard reference ranges and should not replace clinical judgement or formal laboratory reporting. Reference ranges vary slightly between laboratories depending on assay methodology and population calibration; always interpret results in the context of the specific laboratory's reference intervals. TSH alone can be misleading in central (secondary) hypothyroidism, where TSH may be normal or low despite inadequate thyroid hormone production due to pituitary dysfunction. Non-thyroidal illness (sick euthyroid syndrome) can suppress TSH and alter free hormone levels without true thyroid disease. Certain medications — including amiodarone, lithium, glucocorticoids, dopamine, and biotin supplements — can interfere with thyroid assays or directly affect hormone levels. Pregnancy alters TSH reference ranges significantly, with lower normal limits in the first trimester. This tool does not account for thyroid antibodies (anti-TPO, anti-thyroglobulin, TRAb), clinical symptoms, imaging findings, or the patient's medication history. All results should be reviewed by a qualified healthcare professional.
Frequently asked questions
What is the normal TSH range for adults?
The widely accepted normal TSH reference range for adults is 0.4–4.0 mIU/L, as established by the American Thyroid Association. However, some laboratories use slightly different ranges (e.g., 0.35–4.5 mIU/L), and optimal TSH may vary by age and clinical context. Pregnant women have trimester-specific ranges, typically lower than the standard adult range.
Can TSH be normal even if I have a thyroid problem?
Yes, in certain situations. Central hypothyroidism (caused by pituitary or hypothalamic dysfunction) can produce low or inappropriately normal TSH despite insufficient thyroid hormone production. Non-thyroidal illness and certain medications can also cause discordant TSH values. This is why Free T4 and sometimes Free T3 are measured alongside TSH for a complete picture.
What is the difference between Free T4 and Total T4?
Total T4 measures both protein-bound and unbound thyroxine, while Free T4 measures only the biologically active, unbound fraction. Free T4 is generally preferred because it is not affected by changes in thyroid-binding proteins, which can fluctuate with pregnancy, liver disease, oral contraceptive use, or certain medications. Free T4 more accurately reflects the amount of hormone available to target tissues.
Why would TSH be low but Free T4 and Free T3 be normal?
This pattern is called subclinical hyperthyroidism. It can be caused by early Graves' disease, autonomous thyroid nodules, excessive thyroid hormone replacement, or exogenous iodine excess. It may also represent a normal variant in some individuals, particularly the elderly. Subclinical hyperthyroidism is associated with increased risk of atrial fibrillation and bone loss, especially in postmenopausal women, warranting clinical evaluation.
How does the thyroid function calculator help with levothyroxine dosing monitoring?
When a patient is on levothyroxine replacement therapy, the goal is typically to normalise TSH within the reference range while keeping Free T4 in the normal range. This calculator helps quickly assess whether TSH is over-suppressed (suggesting over-treatment) or still elevated (suggesting under-treatment). However, dose adjustments should always be made by a physician based on the full clinical picture, including symptom response and individual treatment targets.
Last updated: 2025-01-15 · Formula verified against primary sources.