Thyroid tests are among the most commonly ordered blood tests, and among the most commonly misunderstood. A slightly high TSH doesn't always need treatment, and a "normal" report doesn't always mean the dose is right. Here is how the tests work.
TSH: the main test
TSH (thyroid stimulating hormone) comes from the pituitary gland in the brain, not from the thyroid. It works like a thermostat signal:
- When the thyroid is underactive, the pituitary pushes harder and TSH goes up.
- When the thyroid is overactive, the pituitary backs off and TSH goes down.
This is why a high TSH means low thyroid function (hypothyroidism), and a low TSH usually means an overactive thyroid. It confuses many patients. In most laboratories, the adult reference range is roughly 0.4 to 4.5 mIU/L, but ranges differ between labs and are different in pregnancy, so always compare with the range printed on your report.
Free T4 and T3
T4 is the main hormone made by the thyroid. T3 is the active form. Free T4 is the most useful second test because it shows how much hormone is actually available to the body. T3 is mainly helpful when checking for an overactive thyroid.
| TSH | Free T4 | Usually means |
|---|---|---|
| High | Low | Hypothyroidism |
| Mildly high | Normal | Subclinical (mild) hypothyroidism |
| Low | High | Hyperthyroidism |
| Low | Normal | Subclinical hyperthyroidism, or recovery after illness |
| Low or normal | Low | A pituitary problem, which needs specialist assessment |
Thyroid antibodies
Anti-TPO antibodies show autoimmune thyroid disease (Hashimoto's thyroiditis), the most common cause of hypothyroidism. A positive result with a mildly high TSH makes it more likely that the thyroid will slowly become underactive. TSH receptor antibodies (TRAb) help diagnose Graves' disease, a common cause of an overactive thyroid.
Does a mildly high TSH need treatment?
Not always. TSH can rise temporarily after an illness, and can be slightly higher in older adults. Often the test is simply repeated after 6 to 8 weeks. Treatment is more likely to help if TSH is above 10, if you are pregnant or planning pregnancy, if antibodies are positive, or if you have clear symptoms.
Getting accurate results
- If you take thyroxine, give blood before your morning tablet, or keep the timing the same each time.
- Biotin (found in many hair, skin and nail supplements) can make thyroid results falsely abnormal. Stop it for a few days before the test, and ask your doctor how long.
- After a dose change, wait 6 to 8 weeks before rechecking TSH, since it takes that long to settle.
- Use the same laboratory where possible, so results are easier to compare.
Taking thyroxine correctly
Take the tablet on an empty stomach with water, 30 to 60 minutes before breakfast or tea. Keep calcium, iron and antacid tablets at least four hours apart from it. Don't stop the tablet or change the dose without advice, even when you feel well.
When to see a thyroid specialist
- Pregnancy, or planning pregnancy, with any thyroid problem
- A low TSH or an overactive thyroid
- A neck swelling or lump
- Results that don't fit together, or symptoms that continue on treatment
- Thyroid problems in children
Book a consultation
Send a WhatsApp message with the patient's name, age and preferred location. You'll get a reply with an appointment time.
Written by Dr. Prasadh Rao L, MBBS, MD (General Medicine), DM (Endocrinology). Assistant Professor of Endocrinology, Madras Medical College. Tamil Nadu Medical Council Reg. No. 96520.
This article is for general information. It is not a diagnosis or a treatment plan. Please don't start, stop or change any medicine without speaking to your doctor.