When are thyroid function tests indicated?

Indications for Thyroid Function Tests

Thyroid function tests are indicated for patients with symptoms suggestive of thyroid dysfunction, those with high-risk factors, and in specific clinical scenarios, but routine population-based screening is not recommended. 1

Symptomatic Patients

Thyroid function testing should be performed in patients presenting with:

• Hypothyroidism symptoms: Fatigue, weight gain, cold intolerance, dry skin, constipation, mental slowing, voice changes 1, 2
• Hyperthyroidism symptoms: Weight loss, palpitations, heat intolerance, anxiety, insomnia, diarrhea, tremors 1, 3
• Physical findings: Goiter, exophthalmos, thyroid nodules, abnormal growth rate 1, 3

High-Risk Populations (Case Finding)

Thyroid function testing is strongly recommended in:

• Women older than 60 years 1
• Patients with previous thyroid surgery or dysfunction 1
• Patients with type 1 diabetes mellitus 1
  • Consider testing children with type 1 diabetes for antithyroid peroxidase and antithyroglobulin antibodies soon after diagnosis 1
  • Measure TSH at diagnosis when clinically stable or after glycemic control is established 1
• Patients receiving immune checkpoint inhibitors 1
• Patients with personal history of autoimmune disease 1
• Patients with family history of thyroid disease 1
• Patients with atrial fibrillation 1
• Patients with previous radiation treatment of the thyroid gland 1
• Postpartum women 1
• Patients with Down syndrome 1
• Pregnant women with risk factors for thyroid disease 1, 4

Monitoring Scenarios

• Patients on levothyroxine therapy:
  • Adults: Monitor TSH 6-8 weeks after dosage changes, then every 6-12 months if stable 4
  • Children: Monitor TSH and free T4 at 2 and 4 weeks after treatment initiation, 2 weeks after any dosage change, then every 3-12 months 4
• Patients with subclinical thyroid disease:
  • TSH persistently <0.1 mIU/L or >10 mIU/L 2
  • TSH abnormalities with elevated thyroid antibodies 2
• Patients receiving immune checkpoint inhibitors:
  • Regular thyroid function monitoring during treatment 1

Testing Approach

1. Initial test: TSH is the preferred initial test (sensitivity 98%, specificity 92% when used for suspected thyroid disease) 1, 5
2. Follow-up testing:
   • If TSH is abnormal, measure free T4 5
   • If TSH is undetectable and free T4 is normal, measure T3 5
   • For suspected hypophysitis, measure morning cortisol, ACTH, gonadal hormones in addition to thyroid function 1

Important Caveats

• TSH values below 0.1 mU/L are considered low and values above 6.5 mU/L are considered elevated 1
• Thyroid function tests may be misleading if performed during acute illness or metabolic decompensation (euthyroid sick syndrome) 1
• Population-based screening for thyroid disease is not recommended due to low positive predictive value in general population 1
• The TRH stimulation test may be useful in patients with symptoms suggesting hypothyroidism but normal baseline thyroid function tests 6

Special Considerations

• Pregnancy: For pregnant patients with pre-existing hypothyroidism, measure TSH and free T4 at pregnancy confirmation and at minimum during each trimester 4
• Children with type 1 diabetes: Recheck thyroid function every 1-2 years if normal initially, or sooner if thyroid antibodies are positive or symptoms develop 1
• Elderly patients: Lower threshold for testing due to higher prevalence and often atypical presentation 1, 2

By following these evidence-based indications for thyroid function testing, clinicians can appropriately identify patients who would benefit from diagnosis and treatment of thyroid dysfunction while avoiding unnecessary testing in low-risk populations.