TSH Interpreter

Why the target depends on your situation

TSH is made by the pituitary and tells the thyroid how hard to work. When thyroid hormone is low, TSH rises to push harder; when it is high, TSH falls. That inverse relationship is why TSH is such a sensitive first test — and also why it is easy to misread out of context.

As a general adult, most laboratories use a range of roughly 0.4 to 4.5 µIU/mL. The exact range varies between laboratories because it depends on the assay and the reference population, which is why a result of 4.6 can be flagged at one lab and normal at another.

In pregnancy, the targets are tighter and change by trimester. Thyroid hormone requirements rise substantially in the first trimester, and untreated hypothyroidism carries real risk for the pregnancy. If you are pregnant or trying to conceive, a TSH that would be acceptable otherwise may well need treating.

On levothyroxine, the number is a treatment target rather than a diagnostic threshold, and the target depends on why you are taking it. Someone treated for hypothyroidism has a different target from someone treated after thyroid cancer.

Units

TSH is reported in µIU/mL in Peru, which is numerically identical to mIU/L, the unit used in the UK and Europe. No conversion is needed — a TSH of 2.1 is a TSH of 2.1 anywhere.

Three things that distort the result

Taking levothyroxine on the morning of the blood draw. Free T4 will read transiently higher than your true steady state. Take your dose after the test.

Biotin supplements. High-dose biotin, common in hair and nail products, interferes with the immunoassays used for thyroid testing — pushing TSH artificially down and free T4 artificially up. Stop it at least 48 hours before testing.

Acute illness. During significant illness, thyroid results can shift in ways that reflect the illness rather than the thyroid. Testing during an acute admission is generally a poor idea unless there is a specific reason.

TSH alone doesn't finish the job

An abnormal TSH is a reason to look further, not a diagnosis. What completes the picture:

• Free T4 — distinguishes overt from subclinical disease, which changes whether treatment is indicated
• Thyroid antibodies (anti-TPO) — tells you whether the cause is autoimmune, and helps predict whether a borderline result will progress
• Ultrasound — when a nodule or goitre is suspected

Subclinical hypothyroidism — a raised TSH with a normal free T4 — is the situation that generates the most disagreement, and reasonably so. Whether to treat depends on how high the TSH is, whether antibodies are present, your age, your symptoms, and whether pregnancy is in the picture. It is a judgement, not a threshold.