Medical Book

LANGUAGE
English Español 中文 Українська
Search

Showing results for: Array

    Medical Book

    LANG
    English Español 中文 Українська
    49 Thyroxine (T4),Total
    Home > Parameters

    49 Thyroxine (T4),Total

    Total thyroxine (T4): thyroid hormone action, reference range, binding proteins and signs of hypo- and hyperthyroidism.

    Thyroxine (T4) is the main hormone secreted by the thyroid gland. It is made from the amino acid tyrosine and iodine. In the tissues, T4 is converted into the more active triiodothyronine (T3). Thyroid hormones regulate metabolism and heat production and act on almost every organ: they speed up the heart and raise its contractility, stimulate breathing, bone turnover, gut motility and brain function. Production is controlled by TSH from the pituitary. More than 99% of T4 in blood is bound to proteins (mainly thyroxine-binding globulin, TBG); total T4 includes both bound and free hormone.

    ThyroidHormonesMetabolism
    Total T4, adults≈ 60–150 nmol/L (≈ 5–12 µg/dL)
    Conversion1 µg/dL ≈ 12.9 nmol/L
    First-line testTSH, then free T4
    Affected bybinding proteins (TBG)
    Total thyroxine scale: below about 60 nmol/L low, about 60 to 150 normal, above about 150 high; reference ranges depend on the laboratory Total thyroxine, T4 (nmol/L) Low below ≈ 60 60 Normal ≈ 60–150 150 High above ≈ 150
    Figure 1. Approximate total T4 zones in adults. Interpret together with TSH and free T4; changes in binding proteins alter total T4 without thyroid disease.

    When total T4 misleads

    Total T4 depends on the amount of binding proteins. Higher TBG (pregnancy, oestrogen-containing pills, hepatitis) raises total T4 while free T4 and thyroid function stay normal. Lower TBG (androgens, glucocorticoids, nephrotic syndrome, liver cirrhosis) lowers total T4. Free T4 and TSH are therefore preferred for diagnosis.

    Clinical significance

    Low T4 (hypothyroidism)

    Autoimmune (Hashimoto) thyroiditis, after thyroid surgery or radioiodine, iodine deficiency, medicines (amiodarone, lithium); rarely pituitary disease. Symptoms: tiredness, weight gain, cold intolerance, dry skin, constipation, slow pulse, low mood.

    High T4 (hyperthyroidism)

    Graves disease, toxic nodules, thyroiditis, excess thyroid hormone tablets, iodine load. Symptoms: palpitations, weight loss, heat intolerance, tremor, anxiety, diarrhoea; risk of atrial fibrillation and osteoporosis.

    Seek emergency care

    for very fast or irregular heartbeat, high fever, confusion or agitation in a person with hyperthyroidism (possible thyroid storm), or extreme drowsiness and low body temperature with hypothyroidism.

    References

    1. Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343–1421. doi:10.1089/thy.2016.0229

    This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

    Published on 1 May 2024