What is a normal TSH level, and why does mine keep changing?
TSH is a message from your pituitary gland to your thyroid. The pituitary sends it, the thyroid receives it, and the number on your report tells you how loudly the pituitary is asking. That number moves with your age, the hour of the blood draw, what you swallowed that week, whether you were sick, and whether estrogen came into or left the picture. One reading is a snapshot. If you are a woman in perimenopause, a man in his fifties, or twenty six and tired, the same rules apply to you.
What TSH is
TSH stands for thyroid stimulating hormone. The name tells you its job. It is made in the pituitary, a small gland at the base of your brain, and it travels in the blood to the thyroid in your neck. It tells the thyroid to make thyroid hormone, mostly T4, which your tissues convert to the active form, T3.
Think of a thermostat and a furnace. The pituitary is the thermostat. The thyroid is the furnace. When thyroid hormone in the blood runs low, the pituitary turns TSH up. When there is plenty, it turns TSH down. This is why a high TSH usually points to an underactive thyroid and a low TSH usually points to an overactive one. People read it backwards all the time, and I understand why.
The pituitary is very sensitive. A small drop in T4 produces a large jump in TSH. That sensitivity is the reason TSH is the first test doctors order. It is also the reason the number is so easy to nudge.
What the lab calls normal
Most labs print a reference range somewhere near 0.4 to 4.5 mIU/L. The exact edges differ from lab to lab, because the range comes from the group of people that lab measured, on that lab's equipment.
The range describes a population. It does not describe you. Each person tends to live in a narrow band of their own inside that wide range. A move from 1.2 to 3.8 never leaves the printed range, and for some people it is a real change. This is why I ask clients to bring every old result they can find. I am looking at the direction over years, and one page cannot show that.
The number tells you how loudly the pituitary is asking. It does not tell you why.
What moves TSH when nothing is wrong
Time of day is the first thing. TSH follows a daily rhythm. It runs highest overnight and in the early morning, and lowest in the afternoon. The same person can read noticeably lower in the middle of the afternoon than first thing in the morning. If one test was drawn before work and the next after lunch, part of the difference is the clock.
Age is the second. TSH tends to drift upward as people get older. In someone past seventy, a reading a little above the top of the range is common and is often left alone. A number that would start a conversation at thirty can mean very little at eighty.
Sex matters for how often things go wrong. The American Thyroid Association notes that women are five to eight times more likely than men to have thyroid problems. Men still get them. In my experience men get tested later, because nobody thinks to look.
Biotin is the one almost nobody is told about. It is in most hair, skin and nail supplements, often at high doses. It interferes with the way many labs run the test. It can make TSH read falsely low and T4 and T3 read falsely high, which looks like an overactive thyroid on paper. The American Thyroid Association advises stopping biotin for at least two days before thyroid bloodwork. Tell whoever orders the test that you take it.
Being sick shifts the number too. A bad flu, surgery, or a hospital stay can push TSH down and then up again for weeks while the body recovers. Unless there is an urgent reason, the cleaner test is the one drawn after you are well.
Medications can move it. Lithium and amiodarone are known to affect the thyroid. Steroid medications at higher doses can lower TSH. Bring your full list, including supplements. Do not stop a prescribed medication to get a tidier result. That decision belongs to the person who prescribed it.
What estrogen does to the picture
Menopause by itself is not known to shift TSH in a big way. What changes is everything around it. Thyroid disease becomes more common in women as the years go on, and the symptoms overlap almost completely with perimenopause: fatigue, weight change, low mood, poor sleep, thinning hair, feeling too hot or too cold. So the thyroid gets blamed for menopause, and menopause gets blamed for the thyroid. A blood test is the only way to tell them apart.
Estrogen taken by mouth changes how thyroid hormone is carried. Oral estrogen, in a birth control pill or in hormone therapy tablets, tells the liver to make more of the protein that carries thyroid hormone in the blood. More carrier means less free hormone for a while. A healthy thyroid makes a little more and you never notice. A thyroid that depends on medication cannot do that. Research in women taking thyroid hormone found that starting oral estrogen can raise the dose they need.
If you take thyroid medication and you start or stop oral estrogen, ask your doctor to recheck your TSH about six to twelve weeks later. Estrogen through the skin, as a patch or gel, has much less of this effect. Hormone therapy remains the most effective treatment for hot flashes, and the choice of type and route is a conversation for a menopause trained doctor.
Pregnancy moves TSH more than anything else on this list. It normally runs lower in the first trimester, and labs use separate ranges for each trimester. If you take thyroid medication and you become pregnant, call your doctor right away. The dose often needs to go up early.
What I would do first
Get the test drawn at the same time of day, every time. Early morning is the simplest habit to keep. Use the same lab when you can, so you are comparing like with like.
Stop biotin at least two days before, and check the labels on your multivitamin and your hair supplement. Many people take it without knowing.
If you take levothyroxine or any thyroid medication, take it the same way each day and do not change the dose yourself. After any dose change, TSH needs about six to eight weeks to settle before a retest means anything.
Ask for the free T4 to be run alongside the TSH, and ask whether thyroid antibodies make sense for you. TSH is the question. Free T4 is the answer the thyroid gave. I wrote about this in Issue 011.
Keep every result. Write the date, the time of the draw, and what you were taking next to each one. A folder with five years in it tells a story a single page cannot.
I do not diagnose, order tests or treat. I read the whole folder with you, and then I send you back to your doctor with better questions.
When to push harder with your doctor
A mildly high TSH on one test is a reason to repeat it, usually in two to three months, with a free T4. Many mildly high readings come back into range on their own. If yours stays high, ask what the plan is. A TSH above 10 is generally treated. Between the top of the range and 10, the decision depends on your symptoms, your antibodies, your age and your heart health, and you are allowed to ask how your doctor is weighing those.
A low TSH deserves the same attention, even when you feel fine. Over time it can affect heart rhythm and bone, especially past sixty five.
Push for a fuller look if you have symptoms and a family history of thyroid or autoimmune disease, if you are trying to conceive or pregnant, if you feel a lump or swelling in your neck, or if your TSH is in range and your free T4 is low. That last pattern can point upstream to the pituitary, and TSH alone will miss it.
A racing or irregular heartbeat, chest pain, or severe new symptoms are not a newsletter question. Get seen that day.
FAQ
What is a normal TSH level?
Most labs use a range near 0.4 to 4.5 mIU/L, and the exact edges vary by lab. Your own normal sits in a narrower band inside that range. A result is best read next to your older results and a free T4.
Does TSH change with age?
Yes. TSH tends to rise slowly as people get older. A reading slightly above range in someone past seventy is common and is often watched and left untreated.
What time of day should I get my TSH tested?
TSH is highest overnight and in the early morning and lowest in the afternoon. The exact hour matters less than keeping it the same each time. Pick a morning slot and stay with it so your results can be compared.
Can biotin affect my thyroid test?
Yes. Biotin can make TSH read falsely low and thyroid hormones read falsely high on many lab methods. The American Thyroid Association advises stopping it at least two days before thyroid bloodwork.
Does menopause or hormone therapy change TSH?
Menopause itself is not known to move TSH much, though thyroid problems get more common with age and feel very similar. Oral estrogen can raise the dose needed in people who take thyroid medication. Ask for a TSH recheck six to twelve weeks after starting or stopping it.
In good health,
Yvette Rose
Sources
- American Thyroid Association. Thyroid function tests.
- American Thyroid Association. Press room: general information about thyroid disease.
- American Thyroid Association. Biotin supplement use can lead to false thyroid test results. Clinical Thyroidology for the Public, December 2018.
- Mazer NA. Interaction of estrogen therapy and thyroid hormone replacement in postmenopausal women. Thyroid, 2004.
This is general wellness education and not medical advice, diagnosis or treatment. Talk to a qualified professional, especially if you are pregnant, nursing, taking medication or managing a condition.