TSH level when losing weight and on medication

The lab measures the amount of TSH in your blood and reports it in milli-units per litre (mE/L). Your pituitary makes TSH and uses it to drive your thyroid, and your thyroid then produces T4 and T3, the two hormones that largely set your resting metabolism.

The coupling is log-linear: a small fall in free T4 lifts TSH sharply, which is why the test picks up an early problem quickly. The direction confuses almost everyone. If your thyroid runs slow, your pituitary pulls harder and the number goes up; if your thyroid makes too much hormone, the signal switches off and the number drops. So a high number fits an underactive thyroid and a low number an overactive one.

For a weight-loss programme there is something else that matters: losing weight itself shifts the axis. Under a longer energy deficit free T3 usually falls first, while TSH can still be normal, and that is your body adapting and not thyroid disease. That is why a baseline before you start is so valuable; without it you cannot tell later whether something changed or was always like that.

Always request free T4, the thyroid hormone itself, in the same tube. Fasting is not needed.

The thyroid almost always comes up when people lose weight, usually with the question of whether an underactive thyroid explains the weight. The honest answer is: partly, and less than you hope. An untreated underactive thyroid usually accounts for a few kilos, a good part of which is fluid rather than fat; it does not explain tens of kilos, and treatment does not make the weight disappear on its own. That does not make measuring pointless, because an underactive thyroid does make losing weight harder, as your energy, your mood and your recovery all suffer.

There is another reason to measure: age. The upper limit of TSH climbs over the years. In large population research the 97.5th percentile rose from 3.56 mE/L in the twenties to 7.49 mE/L over eighty, and an estimated seventy percent of older people above the laboratory limit fell inside their own age-specific range. So a mildly raised value means something different at sixty than at twenty-five.

What happens to your TSH in a programme depends on what is going on. Under a longer energy deficit free T3 often drops first while TSH stays normal. After substantial weight loss the axis resets, so repeat after a few months. Lithium and amiodarone can genuinely disturb the thyroid. A stomach protector, calcium or iron slows the absorption of levothyroxine. And biotin from supplements distorts the measurement itself while your thyroid works normally.

If your value is just above the limit while free T4 is normal, that is called subclinical hypothyroidism. Many people notice nothing and often it does not progress; your doctor weighs your age, your symptoms and a repeat measurement before anything happens.

There are three moments when measuring pays off most in a weight-loss programme. The first is at the start, before you begin: that baseline is later your only real point of comparison, and without it you cannot tell whether an abnormality is new. The second is after a few months, once you have lost a substantial amount and the axis has reset; measuring in the first weeks of a strict deficit mostly reflects the deficit. The third is when symptoms do not fit your programme, such as fatigue that persists while you are eating and sleeping better, or palpitations, heavy sweating and rapid weight loss without doing anything for it.

The time of day counts. Give blood in the morning and around the same hour when repeating, because the value drops through the day. Fasting is not needed.

If you take thyroid medication and your dose just changed, wait 6 to 8 weeks, because measuring sooner gives a number that is still moving.

Always request free T4 alongside. If the value stays raised, anti-TPO is the next logical step, the antibody that shows whether an autoimmune cause is involved.