TSH level

A TSH test measures how much thyroid-stimulating hormone circulates in your blood. TSH comes from the pituitary gland, a small gland at the base of the brain. The pituitary makes TSH and uses it to drive the thyroid. The thyroid then produces the hormones T4 and T3.

The feedback between the two is log-linear, not proportional. A small fall in free T4 produces a much larger rise in TSH. That is what makes TSH the most sensitive single test for thyroid disease. It also explains something that often confuses people. TSH can sit far outside the reference range while free T4 is still normal.

The direction is the opposite of what most people expect. If the thyroid runs slow, the pituitary pulls harder and TSH rises. If the thyroid makes too much hormone, the signal switches off and TSH falls. So a high value points to an underactive thyroid. A low value points to an overactive one.

The sample is an ordinary blood draw from the arm. No fasting is required. The unit appears on reports as mE/L, mU/L or mIU/L. Those are the same unit written three ways. TSH is rarely judged on its own. Free T4 almost always accompanies it, because only the pair completes the picture.

TSH is where thyroid investigation almost always starts. That follows from the log-linear feedback: the pituitary amplifies a small change into a large one. It is also the test most often misread. Four misunderstandings come up again and again.

The first is the direction. Many people read a high value as an overactive thyroid. The opposite is true. A high TSH means the pituitary has to pull harder. That happens precisely when the thyroid produces too little hormone.

The second is the difference between subclinical and overt. Subclinical hypothyroidism is a TSH above the upper limit with a normal free T4. Subclinical hyperthyroidism is a TSH below the lower limit with normal free T4 and free T3. Overt means the thyroid hormone itself is out of range. Those four words decide how a doctor weighs your result.

The third is age. The upper limit of TSH rises over the years. In NHANES III the 97.5th percentile rose from 3.56 mE/L in the twenties to 7.49 mE/L over eighty. Against one fixed laboratory limit, some older people are therefore labelled subclinically hypothyroid without cause. An estimated seventy percent of them fell inside their own age-specific range.

These figures come from research, not from the laboratory's reference column. They explain why a doctor sometimes takes no action on a mildly raised value in an older reader.

The fourth is that TSH is not always usable. If the pituitary itself is diseased, the signal cannot be trusted by definition. TSH also stays suppressed for months after treatment for an overactive thyroid. During serious illness, and with certain interfering substances in the blood, the result is equally unreliable.

TSH is requested for symptoms that fit an underactive or overactive thyroid. Think of persistent fatigue, weight change, feeling cold, palpitations or a shift in mood. It is also a logical first step for an unexplained change in the menstrual cycle.

Timing matters here more than for most blood values. TSH peaks overnight between 02:00 and 04:00 and dips in the early afternoon. Within-person variation across 24 hours is roughly 31 percent. For the thyroid hormones themselves it is 4 to 7 percent.

That difference is clinically visible. In a study of people with subclinical hypothyroidism the median morning TSH was 5.83 mE/L. The same median in the afternoon was 3.79 mE/L. About half of the cases were no longer identifiable as abnormal on the afternoon draw. So when you repeat the test, give blood at roughly the same time of day.

After a dose change in thyroid medication, TSH needs 6 to 8 weeks to reach a new equilibrium. Rechecking sooner returns a number that is still moving.

Pregnancy uses different limits. hCG stimulates the TSH receptor, so TSH falls physiologically in the first trimester.

The 2017 ATA guideline abandons the old 2.5 mE/L cut-off. Where local trimester ranges are missing, the lower limit drops by about 0.4 mE/L and the upper by about 0.5 mE/L. Always discuss an abnormal value in pregnancy with your midwife or doctor.