Read TSH and thyroid hormone results by pattern, and know when thyroid dysfunction changes ED, ICU or transport care and when it doesn't.
The Thyroid Panel (thyroid function tests, TFTs) measures the pituitary signal (thyroid-stimulating hormone, TSH) and the thyroid hormones it controls (free thyroxine [free T4], free triiodothyronine [free T3], and the total T4 and total T3 levels that include protein-bound hormone). The panel has five measured components. TSH is usually the first test because it responds to thyroid hormone changes earlier and more sensitively than the hormones do. Free T4 confirms the thyroid state.
In acute care, a thyroid panel rarely changes the first minutes of management. It matters when a patient has known thyroid disease, takes levothyroxine or an antithyroid drug, or has a clinical picture of thyroid storm or myxedema coma. In those cases the diagnosis is clinical, and the labs confirm it and guide the next day of care. The panel also helps interpret a low or high TSH in critical illness, where thyroid tests are often misleading.
Pituitary signal and the most sensitive screen for thyroid dysfunction in outpatients. High in primary hypothyroidism; low in hyperthyroidism. Unreliable in central disease and many critical illnesses.
Learn MoreUnbound thyroxine, the test that confirms thyroid state with TSH. Low in primary and central hypothyroidism; high in Graves disease, toxic nodules and thyroiditis. Largely independent of binding-protein shifts.
Learn MoreUnbound triiodothyronine. Add it when TSH is low and free T4 is normal, to look for T3 toxicosis. Falls in nonthyroidal illness and is not a screening test.
Learn MoreBound plus free thyroxine. Rises with thyroid-binding globulin (pregnancy, estrogen) even when thyroid function is normal, so free T4 is preferred for diagnosis.
Learn MoreBound plus free triiodothyronine. Rarely needed. Used mainly for T3 toxicosis. Low total T3 in critical illness does not indicate hypothyroidism.
Learn More| Test | Adult Normal (conventional) | SI Units | Critical Low | Critical High | Notes |
|---|---|---|---|---|---|
| TSH | 0.4–4.0 mIU/L | 0.4–4.0 mIU/L | — | — | Lab-specific (Critical Care Transport gives 0.4–4.2 mIU/L); pregnancy-specific ranges |
| Free T4 | 0.8–1.8 ng/dL | 10–23 pmol/L | — | — | Assay-dependent; textbook tables give 0.9–2.3 (Critical Care Transport) and 0.8–1.4 ng/dL (Katzung) |
| Free T3 | about 2.0–4.4 pg/mL | 3.1–6.8 pmol/L | — | — | Assay-dependent; Katzung 15e Table 38-2 lists 2.6–5.7 pmol/L |
| Total T4 | 5.5–12.5 µg/dL | 71–161 nmol/L | — | — | Critical Care Transport Table 18-3; Katzung lists 4.8–10.4 µg/dL. Raised by pregnancy and estrogen |
| Total T3 | 78–158 ng/dL | 1.2–2.4 nmol/L | — | — | MedlinePlus; textbook tables give 60–180 (Critical Care Transport) and 59–156 ng/dL (Katzung). Rarely needed |
Start with serum TSH. If one mildly high value turns up, repeat it, because TSH can rise briefly after acute illness. Treatment is individualized. Many clinicians treat a TSH of 10 mIU/L or higher. For mild elevations in older adults, the evidence for treatment is weak (see Special Populations).
The pattern tells you which test to order next. It is not the final diagnosis.
| Pattern | Key Findings | Common Causes | Next Step |
|---|---|---|---|
| Primary hypothyroidism | TSH high, free T4 low | Hashimoto thyroiditis, post-ablation or post-surgical, iodine excess, missed levothyroxine | Check levothyroxine adherence and interacting drugs; adjust dose |
| Central hypothyroidism | TSH low or normal, free T4 low | Pituitary or hypothalamic disease, glucocorticoid excess | Use free T4 for monitoring; TSH is unreliable |
| Subclinical hypothyroidism | TSH high, free T4 normal | Early Hashimoto disease, recovery from illness, medication effects | Repeat TSH and free T4 in 2–3 months (serial values); individualize treatment |
| Overt hyperthyroidism | TSH low, free T4 high | Graves disease, toxic nodular goiter, thyroiditis, excess levothyroxine | Check thyrotropin receptor antibodies or radioiodine uptake; identify cause |
| Subclinical hyperthyroidism | TSH low, free T4 and free T3 normal | Early Graves disease, nodular goiter, over-replacement, nonthyroidal illness, drugs | Repeat TSH and free T4 in 2–3 months; atrial fibrillation, age 65 and over, or osteoporosis raise the case for treatment |
| T3 toxicosis | TSH low, free T4 normal, free T3 high | Graves disease, toxic nodule, liothyronine use | Order free T3 and clarify T3-containing drugs |
| Sick euthyroid (nonthyroidal illness) | Low free T3, low or normal TSH, free T4 normal or low | Critical illness, fasting, drugs such as high-dose glucocorticoids or dopamine | Do not treat on labs alone; reassess after recovery |
| Amiodarone-induced dysfunction | Hypothyroidism or thyrotoxicosis, with TSH and free T4 changes; hypothyroidism is more common in iodine-sufficient regions, thyrotoxicosis in iodine-deficient regions | Iodine load from amiodarone; type 1 (hyperthyroid) or type 2 (destructive thyroiditis) | Check TSH and free T4 at baseline and periodically on amiodarone (interval per the label and prescriber), and keep checking for months after stopping because of the long half-life; consult prescriber and endocrinology |
| Biotin interference | TSH falsely low; free T4 and free T3 falsely high (assay-dependent) | High-dose biotin supplements | Ask about supplements; hold biotin and repeat the test |
Thyroid hormone levels trail the dose. Free T4 has a half-life of about 7 days. That is why a levothyroxine change takes weeks to show in TSH.
| Analyte | Rises or Falls | Steady State | Repeat Interval | Monitoring Target |
|---|---|---|---|---|
| TSH after levothyroxine change | Slow to change | About 6–8 weeks after a dose change | 6–8 weeks after each change, then every 6–12 months when stable | Lab reference range set by the treating clinician |
| Free T4 after levothyroxine change | Changes over days to weeks | About 6 weeks | With TSH | Central hypothyroidism: upper half of the reference range (draw before the daily dose). Primary hypothyroidism: titrate to TSH; free T4 is a check, not the target. |
| Thyroid storm (therapy) | Free T4 and free T3 fall over days to weeks after antithyroid therapy | Days to weeks | Per endocrinology protocol | Clinical response (heart rate, temperature, mental status) is the first target |
| Myxedema coma (therapy) | Free T4 rises after IV levothyroxine | Days | Per endocrinology protocol during the acute phase | Mental status, temperature, heart rate, respiratory function |
Recognize: Watch for fever, marked tachycardia or atrial fibrillation, agitation or delirium, and vomiting or diarrhea. Then find the precipitant, such as infection, trauma, surgery or iodine load. Grade the severity with a clinical score, such as the Burch-Wartofsky point scale or Japan Thyroid Association criteria, per protocol. The lab values do not grade it.
Follow local protocols and medical direction. Get to a center with endocrine and ICU support early.
Recognize: Altered mental status, hypothermia, bradycardia, hypotension, hypoventilation and hyponatremia. It often shows up in an older adult with untreated hypothyroidism, or after an infection, cold exposure or sedative use. You make this diagnosis at the bedside. Don't wait for labs.
Thyroid analytes are not on standard point-of-care cartridges, such as i-STAT CHEM8+, CG4+ or EG7+. Thyroid results come from the laboratory, every time. Don't use a glucometer or handheld device to estimate thyroid status.
Neonatal and infant TSH values are not adult values. They shift over the first weeks of life. Use the interval your laboratory reports for age. The newborn screen finds congenital hypothyroidism, so follow the state program result. Levothyroxine in children is dosed by weight and age. It is not the adult dose. Follow pediatric endocrinology. See weight-based dosing and pediatric vital signs.
The ATA 2017 guideline calls for population-based TSH reference ranges set by trimester. Total T4 and TSH both change with pregnancy hormones. On levothyroxine, aim for TSH in the lower half of the trimester range. If that range is not available, aim for TSH below 2.5 mIU/L. After a confirmed pregnancy, women on levothyroxine usually need a 20–30% dose increase (for example, two extra tablets per week). In the first trimester, hCG lowers TSH on its own, so read TSH against the trimester ranges. For hyperthyroidism in pregnancy, PTU is the preferred drug in the first trimester. Treat with levothyroxine only. Liothyronine and desiccated thyroid are not recommended in pregnancy. Total T4 rises with estrogen and binding-protein changes, so free T4 is the one to use for diagnosis.
For adults 65 and older with subclinical hypothyroidism (TSH 4.6–19.99 mIU/L and normal free T4), the TRUST trial found no benefit from levothyroxine for symptoms or tiredness at one year. For mild elevations in older adults, individualize the decision. Don't let TSH alone make it. Older adults also run higher TSH values, and overtreatment carries a higher risk of atrial fibrillation.
Acute illness, fasting, glucocorticoids and dopamine can lower T3 and suppress TSH, with no true thyroid disease behind it. Don't start or stop thyroid medication on acute-care labs alone. Reassess after recovery.
| Error or Interferent | Effect on Results | Prevention |
|---|---|---|
| Biotin supplements | Falsely low TSH; falsely high free T4 and free T3 (assay-dependent) | Ask about supplements; pause biotin before testing (duration per laboratory guidance) |
| Heparin plasma | Falsely high free T4 (in vitro free fatty acid generation) | Use serum unless the laboratory validates heparin plasma; interpret with the clinical picture, and repeat off heparin or on another assay if needed |
| Levothyroxine taken before the draw | Free T4 rises briefly after the dose | Draw before the morning dose when monitoring |
| Heterophile antibodies | Falsely abnormal TSH or free hormone results | Interpret with the clinical picture; repeat on another assay |
| Binding-protein changes | Total T4 and total T3 shifted by pregnancy, estrogen or illness | Use free T4 and free T3 for diagnosis |
| Drugs | Levothyroxine absorption reduced by calcium, iron, sucralfate and proton-pump inhibitors; amiodarone and lithium change thyroid function | Separate levothyroxine from calcium, iron and sucralfate by about 4 hours; proton-pump inhibitors act through gastric pH, so recheck TSH if one is started or stopped |