Thyroid Testing in Adults: Recommended Approach and Common Medications
Primary Screening Test
TSH is the single best screening test for thyroid dysfunction in adults, with sensitivity of 98% and specificity of 92% when used to confirm suspected thyroid disease. 1, 2, 3
- Measure TSH as the first-line test for suspected hypothyroidism or hyperthyroidism 2, 4
- TSH values below 0.1 mIU/L are considered low (suggesting hyperthyroidism) 1
- TSH values above 6.5 mIU/L are considered elevated (suggesting hypothyroidism) 1
- The normal reference range is typically 0.4-4.5 mIU/L, though this varies by laboratory 1
Confirmatory Testing Algorithm
If TSH is abnormal, measure free T4 to distinguish between subclinical and overt thyroid dysfunction. 5, 2
For Elevated TSH (Suspected Hypothyroidism):
- Measure free T4 to differentiate subclinical hypothyroidism (normal free T4) from overt hypothyroidism (low free T4) 5
- Confirm with repeat testing after 3-6 weeks, as 30-60% of elevated TSH levels normalize spontaneously 1, 5
- Consider measuring anti-TPO antibodies to identify autoimmune etiology, which predicts 4.3% annual progression risk to overt hypothyroidism versus 2.6% in antibody-negative individuals 5
For Suppressed TSH (Suspected Hyperthyroidism):
- Measure free T4 and T3 to confirm hyperthyroidism 2, 4
- If TSH is undetectable but free T4 is normal, obtain T3 level to detect T3 toxicosis 4
Important Testing Caveats
Do not rely on a single TSH value to establish diagnosis, as TSH can vary by up to 50% on day-to-day basis. 1
- TSH is frequently suppressed during acute illness 1
- Medications (iodine, dopamine, glucocorticoids, octreotide, bexarotene) can affect TSH levels 1
- Pregnancy, anorexia nervosa, and pituitary adenomas interfere with normal TSH levels 1
- Serial TSH measurements are essential to confirm persistent thyroid dysfunction 1
Special Populations Requiring Modified Approach
Elderly Patients:
- 12% of persons aged 80+ with no thyroid disease have TSH >4.5 mIU/L 1
- Standard population reference intervals are inappropriate for older adults 1
Central Hypothyroidism:
- TSH is unreliable; use free T4 to monitor therapy and titrate until free T4 is in upper half of normal range 6, 4
Medications for Thyroid Disorders
Hypothyroidism Treatment:
Levothyroxine (T4) monotherapy is the standard treatment for hypothyroidism. 6, 3
Dosing Guidelines:
- Adults with primary hypothyroidism: Full replacement dose is 1.6 mcg/kg/day 5, 6
- Adults >70 years or with cardiac disease: Start with 25-50 mcg/day and titrate gradually 5, 6
- Patients at risk for atrial fibrillation: Use lower starting dose and slower titration 6
Administration:
- Take as single daily dose on empty stomach, 30-60 minutes before breakfast with full glass of water 6
- Administer at least 4 hours before or after drugs that interfere with absorption (iron, calcium, antacids) 6
Monitoring:
- Check TSH every 6-8 weeks while titrating dose 5
- Once stable, monitor TSH every 6-12 months 5
- Adjust dose by 12.5-25 mcg increments based on TSH results 5, 6
Treatment Thresholds:
- TSH >10 mIU/L: Initiate levothyroxine regardless of symptoms, as this carries ~5% annual risk of progression to overt hypothyroidism 5
- TSH 4.5-10 mIU/L: Consider treatment if symptomatic, pregnant, planning pregnancy, or positive anti-TPO antibodies 5
Hyperthyroidism Treatment:
Therapeutic options include antithyroid drugs, radioactive iodine (currently treatment of choice in US), and thyroidectomy (rarely used). 3
Critical Pitfalls to Avoid
- Never treat based on single abnormal TSH without confirmation testing 5
- Do not use reverse T3 for screening or diagnosis 2
- In suspected central hypothyroidism, always rule out adrenal insufficiency before starting levothyroxine to prevent adrenal crisis 5
- Approximately 25% of patients on levothyroxine are unintentionally overtreated with fully suppressed TSH, increasing risks for atrial fibrillation, osteoporosis, and cardiac complications 5