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Thyroid Imaging in Hypothyroidism

المؤلف:  Wass, J. A. H., Arlt, W., & Semple, R. K. (Eds.).

المصدر:  Oxford Textbook of Endocrinology and Diabetes

الجزء والصفحة:  3rd edition , p548-549

2026-05-07

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Ultrasonography Thyroid ultrasonography may be helpful in determining the cause of hypothyroidism by providing important information on location, size, structure, and vascularity of the gland. In autoimmune thyroiditis a gross inhomogeneity and low echogenicity characterize the echo pattern of the gland. Areas of apparently normal tissue of variable size may be observed, whereas true nodules reflect a different aetiology and should raise the possibility of coexisting nodular goitre, adenomas, or malignancies. A diffuse low thyroid echogenicity is indicative of diffuse autoimmune involvement of the gland and is associated with or may predict the development of hypothyroidism. Studies using colour flow Doppler show a variable degree of vascularity in goitrous autoimmune thyroiditis, whereas vascularity is decreased in the atrophic variant of the dis ease. In subacute thyroiditis the gland is usually enlarged and presents large hypoechoic areas with poorly defined boundaries, mainly within the painful lobe. A large diffuse or multinodular goitre can be documented by ultrasonography in hypothyroidism with inherited defects in thyroid hormone biosynthesis. No evidence of thyroidal tissue in its appropriate location and the demonstration of an ectopic gland are helpful in the diagnosis of congenital hypothyroidism due to thyroid dysgenesis.

In Vivo Isotopic Tests

Thyroid scintiscan may be helpful in the evaluation of hypothyroid patients to indicate the location of functioning thyroid tissue and to provide an estimation of overall thyroid size, although in this regard better evidence is usually obtained by thyroid ultrasonography. Occasionally scintiscan may reveal ectopic thyroid tissue not discernible by other means (e.g. lingual thyroid). Thyroid scintiscan can also be used to reveal substernal thyroid tissue when hypothyroidism is associated with a large goitre.

Radioiodine uptake is expressed as the percentage of radio activity that is trapped by the thyroid at a given time after administration of a tracer quantity of inorganic radioiodine. Early radioiodine uptake measurements (3– 6 h) provide information on the rates of transport and organification of iodide within the gland, whereas 24- and 48- h radioiodine uptake measurement reflects the rate of release of radioiodine from thyroidal tissue. It is also a way of estimating the extrathyroidal pool of iodide, being low to absent after intake of excess iodide but increased in iodine deficiency. An exception is represented by amiodarone- induced hypothyroidism in which radioiodine uptake is preserved despite iodine excess. Radioiodine uptake is increased if hypothyroidism is caused by defective synthesis of thyroid hormones since TSH stimulates all steps in hormone synthesis capable of response. In chronic autoimmune thyroiditis values of the radioiodine up take depend on the amount of residual functioning thyroid tissue and the serum concentration of TSH. Radioiodine uptake may be normal or even increased during the initial phase of chronic thyroiditis, whereas it tends to decrease as the disease progresses. Very low values of the radioiodine uptake are characteristic of the early phase of destructive thyroiditis (e.g. subacute thyroiditis) which is usually associated with thyrotoxicosis caused by follicular disruption. In these cases, return of radioiodine uptake to within the normal range may be helpful to indicate recovery of thyroid function. Radioiodine uptake measurement, which is obviously reduced in postablative hypothyroidism, may be used occasionally to estimate the amount of residual thyroid tissue after thyroidectomy or radioactive treatment.