What tests should be included when ordering a full thyroid panel for a patient, considering their age, sex, and medical history?

How to Order a Full Thyroid Panel

Start with serum TSH alone as your initial test—this is the single most important and cost-effective screening test for thyroid dysfunction, with 98% sensitivity and 92% specificity. 1, 2, 3

Initial Testing Strategy

• Order TSH first in all patients with suspected thyroid dysfunction, regardless of age, sex, or medical history 1, 4, 3
• Do not routinely order free T4, free T3, or "thyroid panels" as initial tests—this leads to unnecessary costs and rarely changes management 5
• TSH alone correctly identifies thyroid dysfunction in the vast majority of cases 4, 3

When to Add Additional Tests

If TSH is Elevated (>4.5-6.5 mIU/L):

• Order free T4 to distinguish between:
  • Subclinical hypothyroidism: elevated TSH + normal free T4 2, 4
  • Overt hypothyroidism: elevated TSH + low free T4 1, 2
• Repeat testing in 3-6 months before initiating treatment, as 30-60% of elevated TSH levels normalize spontaneously 2

If TSH is Suppressed (<0.1 mIU/L):

• Order free T4 first 4, 3
• If free T4 is normal, then order free T3 to detect T3 toxicosis 4
• This confirms hyperthyroidism: suppressed TSH + elevated free T4 and/or free T3 3

If TSH is Normal:

• Stop testing—the patient is euthyroid 3
• No need for free T4 or free T3 in the presence of normal TSH, except in rare cases of central hypothyroidism or TSH-secreting tumors 4, 6

Special Population Considerations

High-Risk Patients Requiring More Frequent Screening:

• Down syndrome patients: More intensive screening recommended, though optimal frequency debated 7
• Elderly women (>60-70 years): Higher prevalence of thyroid dysfunction 2
• Risk factors: Female sex, advancing age, white race (for hypothyroidism), type 1 diabetes, Down syndrome, family history, goiter, previous hyperthyroidism, head/neck radiation, or amiodarone use 1

What NOT to Include in a "Full Panel":

• Avoid routine ordering of:
  • Total T4 and total T3 (influenced by binding protein variations, less useful than free hormones) 8
  • Free T3 as initial test (only indicated when TSH suppressed and free T4 normal) 4
  • Thyroid antibodies as screening tests (not part of initial evaluation) 3
• Studies show that 36% of thyroid function tests ordered are free thyroid hormones when not clinically indicated, wasting approximately $120,000 annually per health system 5

Critical Pitfalls to Avoid

• Never treat based on a single abnormal TSH result—confirm with repeat testing over 3-6 months given high rates of spontaneous normalization 1, 2
• Never order "reflex" thyroid panels—this leads to overdiagnosis and overtreatment 5
• Do not confuse "overt" with "symptomatic"—biochemically "overt" disease (abnormal TSH + abnormal T4) may not have clinical symptoms 1
• Beware of assay interference in patients with discordant results (e.g., elevated TSH with elevated free T4)—consider pregnancy, non-thyroidal illness, medications (amiodarone, heparin), or rare conditions like resistance to thyroid hormone 6

Practical Ordering Algorithm

1. Initial evaluation: Order TSH only 1, 4, 3
2. If TSH abnormal: Add free T4 2, 4
3. If TSH suppressed + normal free T4: Add free T3 4
4. If results discordant with clinical picture: Repeat in 3-6 months, consider confounding factors 1, 6
5. Once diagnosis established: Monitor with TSH alone for primary hypothyroidism 4