TSH (Thyroid Stimulating Hormone)

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TSH (Thyroid Stimulating Hormone) - Comprehensive Medical Test Guide

Why is it done?
- Measures the level of thyroid stimulating hormone produced by the pituitary gland, which controls thyroid hormone production and metabolism
- Screen for hypothyroidism (underactive thyroid) - characterized by fatigue, weight gain, cold sensitivity, and depression
- Screen for hyperthyroidism (overactive thyroid) - characterized by anxiety, weight loss, heat sensitivity, and palpitations
- Diagnose thyroid disorders such as Graves' disease, Hashimoto's thyroiditis, thyroid nodules, and goiter
- Monitor thyroid replacement therapy effectiveness in patients receiving levothyroxine or other thyroid medications
- Routine screening during annual physical examinations, especially in older adults and women over 50
- Evaluate infertility, irregular menstrual cycles, or pregnancy complications
- Investigate symptoms of depression, cognitive changes, or metabolic dysfunction
Normal Range
- Standard Normal Range: 0.4 to 4.0 mIU/L (milliunits per liter)
- Reference Ranges May Vary By: Laboratory testing methodology, pregnancy status (normal range is 0.5 to 2.5 mIU/L during pregnancy), age, medications, and individual patient factors
- Units of Measurement: mIU/L (milliunits per liter) or sometimes reported as mU/L
- Low TSH (Below 0.4 mIU/L): May indicate hyperthyroidism or over-replacement with thyroid medication
- High TSH (Above 4.0 mIU/L): May indicate hypothyroidism or insufficient thyroid hormone replacement therapy
- Borderline Values: Some laboratories consider values between 0.4-2.0 mIU/L or 2.0-4.0 mIU/L as subclinical thyroid dysfunction requiring follow-up evaluation
Interpretation
- TSH Within Normal Range (0.4-4.0 mIU/L): Indicates adequate thyroid function; thyroid gland is producing appropriate levels of thyroid hormones (T3 and T4); patient likely does not have thyroid disease
- Elevated TSH (Above 4.0 mIU/L): Pituitary gland is working harder to stimulate inadequate thyroid hormone production; suggests primary hypothyroidism; pituitary increases TSH signal to compensate for low thyroid hormones; may require thyroid replacement therapy
- Suppressed TSH (Below 0.4 mIU/L): Indicates excess thyroid hormone in the bloodstream; pituitary reduces TSH production as negative feedback; suggests hyperthyroidism or excessive thyroid hormone replacement; requires investigation and possible medication adjustment
- Factors Affecting TSH Results: Time of day (TSH naturally varies with circadian rhythm, typically highest at 2-4 AM and lowest in afternoon), medications (beta-blockers, amiodarone, lithium, corticosteroids, dopamine), pregnancy, recent iodine contrast exposure, severe illness, stress, recent hyperthyroid or hypothyroid events
- Clinical Significance Patterns: High TSH + Low Free T4 = Primary hypothyroidism; Low TSH + High Free T4 = Hyperthyroidism or over-replacement; High TSH + High Free T4 = Thyroid hormone resistance (rare); Low TSH + Low Free T4 = Secondary hypothyroidism (pituitary or hypothalamic disease)
- Subclinical Thyroid Disease: Elevated TSH with normal free T4 may represent subclinical hypothyroidism; suppressed TSH with normal free T4 may represent subclinical hyperthyroidism; both may warrant monitoring and possible treatment
Associated Organs
- Primary Organs Involved: Pituitary gland (anterior lobe - produces TSH), thyroid gland (target organ receiving TSH signal), hypothalamus (releases TRH to stimulate pituitary), and indirectly affects organs dependent on thyroid hormones including heart, brain, liver, and metabolic tissues
- Conditions Associated with High TSH: Hashimoto's thyroiditis (autoimmune thyroid inflammation), iodine deficiency, thyroid hormone insufficiency, thyroid surgery or radioactive iodine treatment complications, medications affecting thyroid function, pituitary or hypothalamic disease (secondary hypothyroidism in rare cases)
- Conditions Associated with Low TSH: Graves' disease (autoimmune hyperthyroidism), toxic nodule or multinodular goiter, thyroiditis (viral or painless), excessive thyroid hormone replacement, amiodarone-induced thyrotoxicosis, pituitary or hypothalamic disease (secondary or tertiary hypothyroidism)
- Diseases Diagnosed/Monitored: Hashimoto's thyroiditis, Graves' disease, thyroid nodules, goiter, thyroid cancer (post-treatment monitoring), thyroid hormonal imbalances, subclinical thyroid dysfunction, central hypothyroidism, pituitary dysfunction
- Potential Complications of Abnormal Results: Untreated hypothyroidism can lead to myxedema coma, cardiovascular disease, cognitive decline, infertility, and metabolic disorder. Untreated hyperthyroidism can cause thyroid storm, atrial fibrillation, heart failure, osteoporosis, and ophthalmologic complications. Excessive thyroid hormone replacement increases risk of atrial fibrillation and accelerated bone loss
- Impact on Other Body Systems: Thyroid hormones affect cardiovascular function (heart rate and contractility), nervous system (mental health, cognitive function), metabolic rate (weight management), bone metabolism, reproductive function, and temperature regulation throughout the body
Follow-up Tests
- Initial Follow-up If TSH Abnormal: Free T4 (thyroxine) level - determines if low TSH is due to hyperthyroidism or secondary hypothyroidism; Total T4 or Free T3 (triiodothyronine) may also be ordered
- Antibody Testing If Autoimmune Disease Suspected: TPO antibodies (thyroid peroxidase antibodies) - indicates Hashimoto's thyroiditis; Thyroglobulin antibodies; TSH receptor antibodies (TRAb) - indicates Graves' disease
- Imaging Studies If Structural Abnormality Suspected: Thyroid ultrasound - evaluates thyroid size, nodules, and echotexture; Thyroid radioiodine uptake scan - determines if high TSH is due to thyroiditis, iodine deficiency, or other causes
- Additional Testing for Low TSH with Normal T4: Free T3 level - determines if subclinical hyperthyroidism is due to T3 thyrotoxicosis; Consider thyroid scan or ultrasound to rule out nodules or thyroiditis
- Testing If Secondary Hypothyroidism Suspected: Pituitary hormone panel (ACTH, prolactin, FSH/LH, GH); MRI pituitary - evaluates pituitary mass or structural abnormality; TRH stimulation test (rarely performed)
- Monitoring Frequency for Treated Thyroid Disease: Every 6-8 weeks after starting or changing thyroid hormone dose until stable; Annually once stable on replacement therapy; More frequently if symptoms persist or dosage adjustments needed; Pregnancy may require more frequent monitoring
- Special Considerations: TSH may be repeated 6-8 weeks after medication dose changes to ensure therapeutic target is achieved; Patients on thyroid cancer treatment require TSH suppression and need frequent monitoring; Post-partum screening recommended as thyroiditis is common after pregnancy
Fasting Required?
- Fasting Requirement: NO - Fasting is not required for TSH testing. Patient may eat and drink normally before the test.
- Timing Considerations: TSH exhibits diurnal variation with highest levels in early morning (2-4 AM) and lowest in afternoon (2-4 PM); For consistent results and proper comparison to previous tests, obtain blood draw at same time of day, preferably morning; Wait at least 4-6 weeks after starting or changing thyroid medication before rechecking TSH
- Medications to Avoid or Note: Do NOT take thyroid medication (levothyroxine, liothyronine) on morning of test; take after blood draw; Inform healthcare provider of all medications including: biotin supplements (can interfere with TSH assay), amiodarone, beta-blockers, lithium, corticosteroids, dopamine, interferon-alpha; Notify provider of recent illnesses, stress, or extreme exercise
- Patient Preparation Requirements: Inform phlebotomist or provider of current medications and supplements; Notify provider of recent iodine contrast exposure (may temporarily affect TSH); Avoid vigorous exercise before test (can temporarily affect TSH); Arrive calm and rested as stress can transiently affect results; Wear loose clothing for easy blood draw access
- Special Population Considerations: Pregnant women should have TSH checked if not already done; Post-menopausal women and older adults may require screening even without symptoms; Recent travelers who consumed excessive iodine should allow time to stabilize before testing

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