Question 1

When should I measure free T3 instead of free T4?

Accepted Answer

Free T3 is primarily useful in suspected T3-toxicosis (hyperthyroidism with suppressed TSH but normal T4), monitoring of anti-thyroid drug therapy, and assessment of amiodarone-induced thyrotoxicosis (type 2).

Question 2

How does illness affect thyroid function tests?

Accepted Answer

Non-thyroidal illness (euthyroid sick syndrome) causes a pattern of low T3, low or normal T4, and normal TSH — mimicking central hypothyroidism. This resolves with recovery of the underlying illness and should not be treated with thyroid hormone.

Question 3

Can biotin supplements interfere with thyroid tests?

Accepted Answer

Yes. Biotin (vitamin B7) interferes with competitive immunoassays commonly used for TSH, T4, and T3, causing falsely low TSH and falsely high free T4/T3. Patients should stop biotin 48–72 hours before testing.

Question 4

What is the significance of an isolated elevated TSH with normal T4?

Accepted Answer

This defines subclinical hypothyroidism. The decision to treat depends on the TSH level (persistent TSH >10 mIU/L often triggers treatment, while TSH 4.5–10 mIU/L is managed based on TPO antibodies, symptoms, cardiovascular risk, and pregnancy planning status).