اختبارات وظائف الغدة الدرقية

• Thyroid function tests are a group of blood tests used to evaluate the function of the thyroid gland
• These tests help diagnose thyroid disorders, such as hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid)

Key Thyroid Hormones

• Thyroxine (T4) and Triiodothyronine (T3) are the main hormones produced by the thyroid gland
• T4 is the primary hormone released, and it is converted to the more active T3 in the body
• Thyroid-stimulating hormone (TSH) is produced by the pituitary gland and regulates thyroid hormone production

Common Thyroid Function Tests

• TSH test measures the amount of thyroid-stimulating hormone in the blood
• Free T4 test measures the level of unbound thyroxine in the blood, which is available to enter tissues
• Total T4 test measures the total amount of thyroxine in the blood, including both bound and unbound forms
• Free T3 test measures the level of unbound triiodothyronine in the blood
• Total T3 test measures the total amount of triiodothyronine in the blood
• Thyroid antibodies tests detect antibodies that attack thyroid tissues, indicating autoimmune thyroid diseases

TSH Test

• Often the first test performed when evaluating thyroid function
• High TSH levels usually indicate hypothyroidism, as the pituitary gland is producing more TSH to stimulate the underactive thyroid
• Low TSH levels usually indicate hyperthyroidism, as the pituitary gland is producing less TSH due to the overactive thyroid

Free T4 Test

• Measures the unbound thyroxine in the blood
• Considered a more accurate reflection of thyroid function than total T4
• Low free T4 levels usually indicate hypothyroidism
• High free T4 levels usually indicate hyperthyroidism

Total T4 Test

• Measures both bound and unbound thyroxine in the blood
• Can be affected by protein levels in the blood, as T4 binds to proteins
• Less commonly used than free T4
• Low total T4 levels can indicate hypothyroidism
• High total T4 levels can indicate hyperthyroidism

Free T3 Test

• Measures the unbound triiodothyronine in the blood
• Useful in diagnosing hyperthyroidism or T3 toxicosis, where T3 levels are disproportionately elevated
• Low free T3 levels can indicate hypothyroidism
• High free T3 levels can indicate hyperthyroidism

Total T3 Test

• Measures both bound and unbound triiodothyronine in the blood
• Similar to total T4, it can be affected by protein levels
• Less commonly used than free T3
• Low total T3 levels can indicate hypothyroidism
• High total T3 levels can indicate hyperthyroidism

Thyroid Antibodies Tests

• Tests for thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb)
• Elevated levels indicate autoimmune thyroid diseases like Hashimoto's thyroiditis (hypothyroidism) and Graves' disease (hyperthyroidism)
• TSH receptor antibodies (TRAb) are specific to Graves' disease
• These antibodies stimulate the TSH receptor, leading to increased thyroid hormone production

Interpreting Thyroid Function Test Results

• Requires consideration of TSH, free T4, and sometimes free T3 levels
• TSH is usually the first test assessed
• Abnormal TSH levels are followed by free T4 testing to confirm the diagnosis
• In some cases, free T3 and thyroid antibody tests are also needed

Hypothyroidism Diagnosis

• High TSH and low free T4 levels indicate primary hypothyroidism (thyroid gland failure)
• Low TSH and low free T4 levels indicate central hypothyroidism (pituitary or hypothalamus dysfunction)
• Elevated thyroid antibodies (TPOAb, TgAb) suggest Hashimoto's thyroiditis

Hyperthyroidism Diagnosis

• Low TSH and high free T4 levels indicate hyperthyroidism
• Low TSH and normal free T4 levels may indicate subclinical hyperthyroidism or T3 toxicosis
• Elevated TRAb confirms Graves' disease
• A thyroid scan may be used to determine the cause of hyperthyroidism

Factors Affecting Thyroid Function Tests

• Medications such as amiodarone, lithium, and corticosteroids can affect thyroid function
• Pregnancy and certain illnesses can alter thyroid hormone levels
• Age can influence normal TSH ranges
• Some herbal supplements can interfere with thyroid function

Preparation for Thyroid Function Tests

• Generally, no fasting is required
• Inform the healthcare provider about all medications and supplements being taken
• Certain medications may need to be temporarily discontinued
• Testing is typically avoided during acute illness if possible

Clinical Significance of Thyroid Function Tests

• Diagnose thyroid disorders
• Monitor treatment effectiveness for thyroid conditions
• Screen for thyroid dysfunction in high-risk individuals

Newborn Screening

• Thyroid function tests, specifically TSH, are part of routine newborn screening
• Early detection and treatment of congenital hypothyroidism can prevent developmental delays

Subclinical Thyroid Disorders

• Subclinical hypothyroidism is characterized by elevated TSH and normal free T4 levels
• Subclinical hyperthyroidism is characterized by low TSH and normal free T4 levels
• Management depends on TSH levels, symptoms, and risk factors

Central Hypothyroidism

• A rare condition resulting from pituitary or hypothalamic dysfunction
• Characterized by low TSH and low free T4 levels
• Requires evaluation of other pituitary hormones

Euthyroid Sick Syndrome

• Abnormal thyroid function tests in the setting of severe non-thyroidal illness
• TSH, T4, and T3 levels may be low or normal
• Thyroid hormone replacement is generally not indicated

Thyroid Function Tests in Pregnancy

• Pregnancy affects thyroid hormone levels, requiring different reference ranges
• Untreated thyroid disorders during pregnancy can have adverse effects on both mother and fetus
• TSH is typically monitored closely throughout pregnancy

Follow-Up Testing

• Periodic monitoring of thyroid function is necessary for individuals on thyroid hormone replacement
• Frequency of testing depends on the stability of thyroid hormone levels and individual patient factors
• Adjustments to medication dosage may be required based on test results

Considerations for Elderly Patients

• TSH reference ranges may be higher in older adults
• Symptoms of thyroid disorders can be atypical in the elderly
• Careful interpretation of thyroid function tests is necessary

Interference with TSH Assays

• Heterophile antibodies and other interfering substances can cause falsely elevated or decreased TSH levels
• Serial dilutions and repeat testing may be necessary to confirm results
• Utilizing different assay platforms can help resolve discrepancies

Emerging Thyroid Function Tests

• Reverse T3 (rT3) is an inactive form of T3, but its clinical utility is controversial
• Serum thyroglobulin (Tg) is used as a tumor marker for differentiated thyroid cancer
• Calcitonin is used to detect medullary thyroid cancer