Thyroid Function Tests

Thyroid function tests assess the hypothalamic-pituitary-thyroid axis. Thyrotropin (TSH) from the pituitary stimulates the thyroid to release thyroxine (T4) and a smaller amount of triiodothyronine (T3).

Most T4 is protein bound, and the free fractions (free T4, free T3) are the active hormones. T4 is converted to T3 in peripheral tissues.

Because TSH changes exponentially with small changes in free T4 (a two fold change in free T4 produces roughly a 100-fold change in TSH), it is the most sensitive test of thyroid status in a patient with an intact pituitary. All are measured by immunoassay.

The panel diagnoses hypothyroidism and hyperthyroidism, separates primary from central disease, monitors replacement and antithyroid therapy, and, with antibodies, finds the cause.

Overt hypothyroidism affects about 1 to 2% of women and 0.1 to 0.5% of men, and subclinical hypothyroidism 4 to 10% of adults, rising with age. Hyperthyroidism affects about 0.5 to 2% of adults, with Graves disease the commonest cause. Thyroid disease is 5 to 10 times more frequent in women.

Indications. Fatigue, weight change, palpitations, tremor, heat or cold intolerance, goitre or nodule, atrial fibrillation, unexplained hyperlipidaemia, hyponatraemia, subfertility, menstrual disturbance, depression, delayed growth, monitoring of treatment, amiodarone or lithium therapy, and neonatal screening.

Specimen. Serum or heparin plasma. Fasting is not needed. Free T4 is best taken before the morning levothyroxine dose when monitoring treatment. Biotin supplements (over 5 mg daily) interfere and are stopped 2 days before sampling.

Ranges are assay specific, and local ranges take precedence.

TSH. Range: 0.4 to 4.0 mIU/L in adults (the upper limit is higher in older adults and lower in the first trimester of pregnancy). Raised: primary hypothyroidism, subclinical hypothyroidism, recovery from non-thyroidal illness, thyroid hormone resistance, TSH-secreting adenoma, adrenal insufficiency, and assay interference. Lowered: hyperthyroidism, central hypothyroidism, first trimester of pregnancy, non-thyroidal illness, glucocorticoids, dopamine, and excess levothyroxine.

TSH is the first line test. A normal TSH excludes primary thyroid dysfunction in almost all patients with an intact pituitary and no recent change in treatment.

Free T4. Range: 9 to 25 pmol/L (0.7 to 1.9 ng/dL). Raised: hyperthyroidism, excess levothyroxine, heparin effect (in vitro lipolysis), amiodarone (variable), and familial dysalbuminaemic hyperthyroxinaemia. Lowered: hypothyroidism, central hypothyroidism, non-thyroidal illness, and severe protein binding changes.

Free T3. Range: 3.5 to 6.5 pmol/L (2.3 to 4.2 pg/mL). Raised: hyperthyroidism (a sensitive marker, and the only abnormality in T3 toxicosis), and rarely in thyroid hormone resistance. Lowered: non-thyroidal illness (low T3 syndrome), hypothyroidism (late), fasting, and amiodarone. It is not used for hypothyroidism.

Thyroid peroxidase antibodies (TPO). Positive in 90% of Hashimoto thyroiditis and 70% of Graves disease, and in 10 to 15% of healthy women. They predict progression of subclinical hypothyroidism (4.3% a year with positive antibodies against 2.6% without) and miscarriage risk.

TSH receptor antibodies (TRAb). Stimulating antibodies cause Graves disease. Positive in 90 to 95% of untreated Graves disease. Used to confirm the cause, to predict relapse after antithyroid drugs, and in pregnancy to assess the risk of foetal and neonatal thyrotoxicosis (when levels exceed 3 times the upper limit).

Thyroglobulin. Produced by thyroid tissue. Used as a tumour marker after total thyroidectomy and radioiodine for differentiated thyroid cancer (undetectable expected), always with thyroglobulin antibodies, which interfere. Raised in thyroiditis, goitre and Graves disease, and low in factitious thyrotoxicosis.

Patterns.

• High TSH, low free T4: primary hypothyroidism.
• High TSH, normal free T4: subclinical hypothyroidism. Repeat in 2 to 3 months. Treatment is considered for TSH over 10 mIU/L, positive antibodies, symptoms, pregnancy or planning pregnancy, and goitre.
• Low TSH, high free T4 or free T3: primary hyperthyroidism.
• Low TSH, normal free T4 and T3: subclinical hyperthyroidism, which carries a risk of atrial fibrillation and bone loss when TSH is under 0.1 mIU/L.
• Low or normal TSH with low free T4: central hypothyroidism (pituitary or hypothalamic disease), which needs pituitary assessment.
• High TSH with high free T4: TSH-secreting adenoma, thyroid hormone resistance, or assay interference.
• Low T3 with low or normal T4 and normal or low TSH in an acutely ill patient: non-thyroidal illness. TSH may fall to under 0.1 mIU/L and recover to a transient rise over the normal range. Thyroid tests are repeated after recovery.

Causes of hypothyroidism. Hashimoto thyroiditis, post-thyroidectomy or radioiodine, drugs (amiodarone, lithium, immune checkpoint inhibitors, tyrosine kinase inhibitors), iodine deficiency, post-partum and subacute thyroiditis (transient), neck irradiation, and congenital.

Causes of hyperthyroidism. Graves disease, toxic multinodular goitre, toxic adenoma, thyroiditis (subacute, post-partum, silent), amiodarone, excess iodine, exogenous levothyroxine, struma ovarii, and hCG-mediated (gestational and molar pregnancy).

Monitoring. On levothyroxine: TSH 6 to 8 weeks after any dose change, then every 12 months once stable. On antithyroid drugs: free T4 and free T3 every 4 to 6 weeks (TSH remains suppressed for months). Central hypothyroidism: free T4 guides treatment, not TSH.

Pregnancy. Reference limits change: TSH upper limit about 3.0 to 4.0 mIU/L in trimesters 2 and 3 and lower in trimester 1. Total T4 and T3 rise (about 1.5 times). Overt hypothyroidism is treated and levothyroxine doses rise by 25 to 30%.

Critical values and findings needing immediate action.

• Free T4 very high with fever, tachycardia, agitation, delirium, vomiting or heart failure, which indicates thyroid storm and needs critical care.
• Severe hypothyroidism (TSH over 50 mIU/L or free T4 very low) with hypothermia, bradycardia, hypoventilation, hyponatraemia or reduced consciousness, indicating myxoedema coma.
• Thyrotoxicosis with atrial fibrillation or heart failure, or with agranulocytosis on antithyroid drugs.
• Abnormal TSH in a pregnant woman or newborn.

Interferences and pitfalls.

• Non-thyroidal illness distorts all tests, and testing in acute illness is deferred unless thyroid disease is suspected.
• Biotin, heterophile antibodies and anti-streptavidin or anti-ruthenium antibodies cause false results in some immunoassays (typically high free T4 and low TSH with biotin).
• Macro-TSH (TSH bound to immunoglobulin) gives a persistently raised TSH with normal free T4.
• Heparin, furosemide, phenytoin and carbamazepine affect free T4 methods and total T4.
• Oestrogens, pregnancy and tamoxifen raise binding protein, and androgens and nephrotic syndrome lower it. Free hormone results correct for this in most cases.
• Levothyroxine taken just before the sample raises free T4 for hours.
• Iron, calcium and proton pump inhibitors reduce levothyroxine absorption.
• Recent iodinated contrast, amiodarone and glucocorticoids alter results.

Special groups.

• Neonates and infants: screening by heel prick TSH or T4 (at day 5), with a TSH over 20 to 40 mIU/L needing urgent confirmation. Age specific ranges are needed.
• Children: delayed growth and puberty suggest hypothyroidism, and congenital hypothyroidism needs treatment within 2 weeks.
• Pregnancy: trimester specific ranges and early correction of hypothyroidism are needed.
• Older adults: TSH upper limit rises (up to about 6 to 7 mIU/L over 80 years), and subclinical hypothyroidism is treated less often.
• Regions: iodine deficiency causes goitre and hypothyroidism, and iodine excess causes hyperthyroidism and thyroiditis.

Follow up tests. Repeat TSH and free T4, TPO and thyroglobulin antibodies, TRAb, free T3, thyroglobulin, thyroid ultrasound, technetium or iodine uptake scan, fine needle aspiration of nodules, pituitary hormones and MRI in central disease, cortisol, lipid profile, creatine kinase, full blood count, and calcitonin when medullary carcinoma is suspected.