Question 1

What does a high TSH with a normal free T4 mean?

Accepted Answer

A pattern of elevated TSH with normal free T4 is **subclinical hypothyroidism**. Treatment is recommended when TSH is greater than 10 mIU/L, the patient is pregnant or planning pregnancy, has clear hypothyroid symptoms, or has positive anti-TPO antibodies. For modestly elevated TSH (5–10) in older adults without symptoms, monitoring rather than treating is reasonable.

Question 2

What is sick euthyroid syndrome?

Accepted Answer

Non-thyroidal illness syndrome (NTIS) is the constellation of altered thyroid lab values seen in acute illness. Free T3 typically falls disproportionately, free T4 may fall in severe illness, and TSH can be low, normal, or transiently elevated during recovery. It is not true thyroid disease — testing should be deferred until recovery whenever possible, and treatment is not generally indicated.

Question 3

How do I differentiate primary from central hypothyroidism?

Accepted Answer

Primary hypothyroidism shows elevated TSH with low free T4. Central (secondary) hypothyroidism shows inappropriately normal or low TSH with low free T4 — the pituitary fails to mount a TSH response to low circulating thyroid hormone. Central disease requires pituitary axis workup (cortisol, prolactin, gonadotropins) and pituitary MRI.

Question 4

Why treat adrenal insufficiency before hypothyroidism?

Accepted Answer

In central hypothyroidism, ACTH deficiency often coexists. Starting levothyroxine before replacing cortisol can accelerate cortisol metabolism and precipitate adrenal crisis. The convention is to confirm and treat adrenal insufficiency first, then start thyroid hormone replacement.

Question 5

How does pregnancy change thyroid testing?

Accepted Answer

TSH ranges fall in pregnancy due to hCG cross-stimulation of the TSH receptor. Trimester-specific reference ranges apply: first trimester upper limit is typically 2.5 mIU/L, climbing modestly in later trimesters. Free T4 ranges shift downward; total T4 (with TBG correction) may be more reliable. Subclinical hypothyroidism is treated more aggressively in pregnancy than outside it.

Question 6

Can biotin interfere with thyroid testing?

Accepted Answer

Yes — high-dose biotin (5–10 mg/day, found in many over-the-counter "hair, skin, and nails" supplements) interferes with streptavidin-biotin immunoassays. The classic interference pattern mimics Graves disease: low TSH with elevated free T4 and free T3. Hold biotin for at least 48 hours before testing if interference is suspected.

Question 7

When should I recheck TSH after a levothyroxine dose change?

Accepted Answer

TSH lags 4–6 weeks behind a dose change because of the long half-life of T4 and the time required for the pituitary to re-equilibrate. Rechecking sooner than 4 weeks risks adjusting based on stale information; 6 weeks is the most common interval cited in guidelines.

Pattern	TSH	Free T4
Primary hypothyroid	↑	↓
Subclinical hypothyroid	↑	N
Primary hyperthyroid	↓	↑
Subclinical hyperthyroid	↓	N
Central hypothyroid	N or ↓	↓
Sick euthyroid	↓ / N / ↑ (transient)	N or ↓

TSH / Free T4 / T3 Pattern Interpreter

References

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