What is TSH?
TSH stands for thyroid stimulating hormone. Despite the name, it isn't made by your thyroid. It's made by the pituitary gland in your brain.
Think of TSH as a message from your brain to your thyroid. When your thyroid hormone levels drop, your brain raises its voice and sends more TSH, basically shouting, "Work harder!" When thyroid hormone is high, your brain quiets down and sends less.
That's why TSH reads backwards from what most people expect:
- High TSH usually means your thyroid is underactive
- Low TSH usually means your thyroid is overactive, or you're taking more thyroid medication than you need
What is the normal TSH range?
Most labs in Ontario flag TSH as normal somewhere between roughly 0.3 and 4 to 5 mIU/L. The exact cut-offs depend on the lab.
Here's the catch. Those ranges come from large groups of people, and some of them already have early thyroid disease without knowing it. When researchers screened out anyone with thyroid antibodies or thyroid symptoms, more than 95% of healthy people had a TSH below 2.5. That's a big gap between "normal" and what's typical in a healthy thyroid.
TSH also drifts up naturally as we age. A TSH that's a concern at 30 may be perfectly fine at 80.
Normal vs optimal TSH levels for women
A normal range is designed to catch disease. An optimal range is where most people actually feel their best. These are the TSH targets I generally work toward with patients:
- Not on medication, negative antibodies: 0.5 to 2.5 mIU/L
- Not on medication, positive antibodies (Hashimoto's): 0.5 to 2.0 mIU/L
- On thyroid medication: 0.5 to 2.0 mIU/L
- Trying to conceive: no higher than 2.0 mIU/L
- Ages 65 to 80: up to about 6.0 mIU/L can be appropriate
- Over 80: up to about 7.5 mIU/L can be appropriate
These aren't rules for everyone. Your symptoms, your other labs and your history all matter. Still, if your TSH is 4.2 and you feel awful, "it's normal" shouldn't be the end of the conversation.
What does high TSH mean?
A high TSH means your brain is working hard to push a thyroid that isn't keeping up. In Canada, the most common cause is Hashimoto's, an autoimmune condition where the immune system targets the thyroid.
Other things that can raise TSH include:
- Recovering from a serious illness
- Certain medications, like lithium or amiodarone
- Missing doses of thyroid medication
- Too much iodine from supplements, especially with Hashimoto's
High TSH levels symptoms
These often creep in slowly. Common ones include fatigue that sleep doesn't fix, brain fog, feeling cold, constipation, dry skin, hair thinning, low mood and heavier periods. I cover the ones that get missed in hypothyroidism symptoms in women.
High TSH with normal T4
This is called subclinical hypothyroidism. Your thyroid is still making enough hormone, but your brain has to push harder to get it. It's often an early sign of Hashimoto's, so this is exactly when I want to check thyroid antibodies. Whether it needs treatment depends on how high TSH is, your symptoms, your antibodies and whether you're planning a pregnancy.
What does low TSH mean?
A low TSH means your brain is barely asking your thyroid for anything. That usually happens when there's already plenty of thyroid hormone around.
Common causes of low TSH include:
- An overactive thyroid, such as Graves' disease or a nodule making extra hormone
- The early, "leaky" phase of thyroiditis
- Taking more thyroid medication than you need
- Early pregnancy, when pregnancy hormones can lower TSH
- High-dose biotin, which can make some TSH tests read falsely low
Low TSH symptoms
Think of the opposite of low thyroid. A racing or pounding heart, anxiety, trouble sleeping, feeling hot and sweaty, shaky hands, weight loss without trying and more frequent bowel movements.
Low TSH with normal T4
This is called subclinical hyperthyroidism. It can be early thyroid overactivity, or a sign your medication dose is a bit high. People on T3-containing medications like Cytomel or desiccated thyroid also tend to run a lower TSH. Please don't change your dose based on this. Bring it to the person who prescribes your medication.
Not sure what your TSH is telling you? Book a free consult and we'll look at your labs together. JOSIE is virtual and sees patients anywhere in Ontario.
Why TSH alone isn't enough
TSH is the messenger, not the whole conversation. On its own, it can't tell you:
- How much hormone your thyroid is actually making. That's free T4.
- How well you're converting it to the active form. That's free T3.
- Whether your immune system is involved. That's anti-TPO and anti-TG antibodies.
This is where Hashimoto's and hypothyroidism get mixed up. Hypothyroidism describes a low hormone level. Hashimoto's describes the autoimmune cause. Antibodies can be raised for years while TSH looks perfectly normal, which is why I always want them tested.
I also like to look at the things that affect how your thyroid works. Ferritin, since your thyroid needs iron to make hormone. Fasting insulin, which is rarely run by family doctors even though insulin resistance and thyroid problems often travel together. Vitamin D and B12 too. More on this in why your thyroid labs look normal but you still feel off and on our specialized testing page.
How to get the most accurate TSH blood test
TSH isn't fixed. It rises overnight and tends to drop through the day and after eating. A few small habits make your results much easier to compare over time:
- Test in the morning, fasted. Aim for around 8 a.m., about 12 hours after your last meal.
- Test before your thyroid medication that day. Take it right after your blood draw.
- Stop high-dose biotin for 7 days before. That includes hair, skin and nail supplements over 10,000 mcg.
- Use the same lab when you can. Different labs use slightly different ranges.
- Skip testing when you're sick. Acute illness can throw TSH off temporarily.
If you're already on thyroid medication
For many people on levothyroxine, TSH is the main number used to adjust the dose. If your TSH is out of range, or in range but you still feel unwell, talk to the doctor who prescribes your medication. Never change your dose on your own. If you're curious about the different options, I break them down in levothyroxine, liothyronine or NDT.
Naturopathic visits aren't covered by OHIP, and that's what gives us time to go through every number on your requisition, not just the one flagged in bold. If you have extended health benefits, there's a good chance naturopathic care is included, and JOSIE direct bills most insurers for you. Learn more about how we approach hypothyroidism and Hashimoto's.
The bottom line
TSH is a helpful starting point, but it's only one piece. If your TSH is "normal" and you still don't feel like yourself, ask for the full thyroid panel and a closer look at what's going on underneath.
