Thyroid
Thyroid disorders
An abnormal TSH is one of the commonest results people arrive with — and one of the most often over-treated. Reading it properly matters.
How the tests fit together
The pituitary gland releases TSH to tell the thyroid to work harder. So TSH moves in the opposite direction to thyroid function: when the thyroid is underactive, TSH rises; when it is overactive, TSH falls. This inversion is the source of most of the confusion patients arrive with.
TSH is the sensitive first test. Free T4 tells you how far the gland has actually drifted, and separates overt disease from the subclinical form where TSH is abnormal but T4 is still normal. Free T3 is useful in suspected overactivity and largely unhelpful otherwise. Antibodies — anti-TPO, and TRAb in overactivity — tell you the cause rather than the severity.
Why thyroid and diabetes turn up together
Autoimmune thyroid disease is markedly more common in people with type 1 diabetes, because the same immune tendency drives both. Anyone with type 1 should have thyroid function checked at diagnosis and periodically thereafter.
The link with type 2 is looser but real, and it runs in both directions: an underactive thyroid raises cholesterol and weight and worsens insulin resistance, while an overactive thyroid can push glucose up and make readings erratic. If control has become unexplainably difficult, thyroid function is worth checking before the diabetes regimen is rebuilt.
When an abnormal result does not need treatment
A mildly raised TSH with a normal free T4 is common, and a proportion of these return to normal on their own. Any acute illness can disturb thyroid tests, which is why a result taken during or shortly after an admission should not be acted on. Biotin supplements — often in hair and nail products — interfere with the assay and can produce a result that looks like overactivity in someone perfectly well.
The habit of starting thyroxine on a single borderline TSH is widespread and does harm: over-treatment carries its own risk of atrial fibrillation and bone loss. Except in pregnancy, where the thresholds are different and the urgency is real, the correct response to one borderline result is usually to repeat it in six to twelve weeks with antibodies, not to start a tablet.
Reading the pattern
Reference ranges differ between laboratories — always read your result against the range printed on your own report.
| Pattern | TSH | Free T4 | Usually means |
|---|---|---|---|
| Normal | Normal | Normal | No thyroid disorder |
| Overt hypothyroidism | High | Low | Underactive; treatment indicated |
| Subclinical hypothyroidism | High | Normal | Mild; repeat before treating |
| Overt hyperthyroidism | Low | High | Overactive; find the cause |
| Subclinical hyperthyroidism | Low | Normal | Repeat; assess heart and bone risk |
A low TSH with a low free T4 does not fit the pattern above and suggests the problem is in the pituitary rather than the thyroid. That is uncommon and needs different investigation.
Common questions
- Do I need to fast for a thyroid test?
- No. But TSH is a little higher early in the morning and drifts down through the day, so if you are being followed on treatment, keep your tests at roughly the same time of day and at the same laboratory. On thyroxine, take the tablet after the blood is drawn, not before.
- Should everyone be screened?
- Not everyone. Testing is reasonable if you have symptoms, a goitre, type 1 diabetes or another autoimmune condition, a strong family history, are pregnant or planning to be, or are on amiodarone or lithium. Routine screening of people with no symptoms and no risk factors mostly produces borderline results that lead to unnecessary treatment.
- Is my neck swelling a thyroid problem?
- Sometimes. A diffusely enlarged thyroid may be autoimmune or, historically in parts of northern Pakistan, iodine-related. A discrete lump is a different question and needs an ultrasound, and a fine-needle sample if the ultrasound features warrant it. A lump that is enlarging, hard, or accompanied by a hoarse voice should be examined without delay.
Important