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A TSH test measures the level of thyroid-stimulating hormone in your blood. Abnormal TSH levels typically indicate a problem with your thyroid, like hypothyroidism or hyperthyroidism. In rare cases, it could indicate problems with your pituitary gland.
The pituitary gland in your brain produces TSH, which tells the thyroid (a gland in your throat) to produce thyroid hormones. Thyroid hormones regulate metabolism, weight, muscle mass, heart rate, and mood.
Healthcare providers in primary care, internal medicine, endocrinology (hormone specialists), obstetrics or gynecology (women’s health), or pediatrics (children’s health) may order TSH levels.
TSH is a chemical messenger that works like a thermostat, signaling your thyroid to increase or slow down thyroid hormone production. When you have low thyroid hormone levels, your pituitary gland releases more TSH, boosting production. When thyroid levels are sufficient, the pituitary gland reduces TSH to slow thyroid hormone production.
Thyroid hormones regulate metabolism, weight, muscle mass, heart rate, and mood. TSH levels that are too high or too low could indicate a problem with your thyroid or pituitary gland.
Your providers may order a TSH test if you have symptoms of an over- or under-active thyroid, symptoms of a pituitary disorder, or a lump on your thyroid. These tests are even used to monitor medications.
Testing for Hypothyroidism
Hypothyroidism (underactive thyroid) is when you don’t produce enough thyroid hormones, slowing these body functions down and causing symptoms like:
- Brain fog
- Cold sensitivity
- Constipation
- Depression or anxiety
- Dry, thinning hair or skin
- Dull facial expressions
- Fatigue
- Irregular menstrual periods
- Large thyroid gland
- Muscle pain
- Paleness or jaundice (yellowing of the skin)
- Prolonged ankle reflex time
- Puffiness
- Slow heart rate
- Slowness of speech
- Sore throat or neck pain
- Weight gain
Testing for Hyperthyroidism
Hyperthyroidism (overactive thyroid) is when you produce too much thyroid hormone, speeding up these body functions. It causes symptoms like:
- Anxiety
- Diarrhea
- Eye changes
- Fatigue
- Fever
- Heat sensitivity (or excess sweating)
- Heart failure
- Hypertension (high blood pressure)
- Hyperreflexia (faster reflexes)
- Irritability
- Large thyroid
- Muscle weakness
- Neck pain
- Rapid or irregular heartbeat
- Shortness of breath
- Tremors (shaky hands)
- Trouble sleeping
- Weight loss, even if your appetite is more significant and you’ve been eating more
A TSH level is typically a good first step or screening tool because changes in TSH occur earlier than changes in the primary thyroid hormones—triiodothyronine (T3) and thyroxine (T4).
Evaluating T3 and T4 levels, thyroid antibodies, and TSH levels together can help your provider determine if the issue is primary (with the thyroid) or secondary (originating in the pituitary gland).
Triiodothyronine (T3) and Thyroxine (T4) Tests
A T3 measures triiodothyronine, the active form of thyroid hormone. T3 makes up about 20% of the total hormones the thyroid releases, and it is more potent than T4. T4 levels measure thyroxine, which accounts for over 80% of the total hormones the thyroid releases. T4 transforms into T3 in the bloodstream.
Free T3 and Reverse T3 Tests
Some healthcare providers also check free T3 and reverse T3 levels. Free T3 measures the active thyroid hormones in your blood. The reverse T3 (rT3) test looks for an inactive hormone.
Thyroid Antibody Tests
Thyroid antibody tests check for antibodies that attack healthy thyroid tissue, causing problems with thyroid function. These tests help providers diagnose autoimmune thyroid diseases. This includes Hashimoto’s thyroiditis, the most common cause of hypothyroidism, and Graves’ disease, the most common cause of hyperthyroidism.
Before the test, tell your provider about any medications or supplements you take. They will tell you if you should stop taking them beforehand.
Wear comfortable clothing with short or loose sleeves for easy access to your arm for the blood draw. Blood draws are usually safe but stressful, so consider bringing someone for support.
Bring your ID and insurance card (if you have one). Most insurance plans cover a TSH test, but check with your provider or insurance company. If you need financial help, ask your healthcare team about assistance programs.
A TSH test involves drawing a small blood sample, usually at a healthcare provider’s office, clinic, or lab. The process is quick, taking just a few minutes. You may need to complete some paperwork beforehand, like your medical history and insurance information.
Ask your healthcare provider if you need to fast (avoid eating or drinking anything except water) before the test. Fasting isn’t usually necessary for a TSH test, but it might be if you’re having other blood tests done.
During the Test
During the test, you’ll sit in a chair while a healthcare professional draws blood from a vein in your arm using a small needle. You might feel a slight sting when the needle goes in or out. The person drawing your blood will:
- Place a wide rubber band on your upper arm
- Clean your skin with alcohol
- Insert a small needle into your vein
- Draw blood into a tube
- Remove the needle
- Apply pressure and a bandage
- Send the blood sample to a laboratory
After the Test
After a TSH blood test, you can usually leave right away. If needles make you queasy, take a moment to rest before driving. After reviewing the results, your provider may suggest further testing, medication changes, or specialist referrals.
Risks and Precautions
Getting any blood test is usually safe. You might feel a little pinch or notice a bruise where the needle was, but it usually goes away quickly.
Results are typically available within a few days. Your healthcare provider will share the results through a patient portal, phone call, text, letter, or follow-up appointment.
Interpreting Your Results
Low or high TSH levels may indicate a thyroid disorder (overactive or underactive).
TSH levels can fluctuate, even in a healthy thyroid. In people over 80, TSH levels are often higher without thyroid problems. Pregnancy or hormonal birth control can also impact TSH levels.
Here’s a quick guide to interpreting TSH results.
| Result | Range | Interpretation |
|---|---|---|
| Normal | 0.4 – 4.5 mlU/L | A TSH level within the normal range generally indicates that your thyroid is functioning correctly. |
| High | >4.5 mlU/L | High TSH levels could indicate too little thyroid hormone, which typically occurs with primary hypothyroidism. |
| Low | <0.4 mlU/L | Low TSH levels may mean your thyroid produces too much hormone, indicating hyperthyroidism. It could also mean that you have secondary hypothyroidism due to a benign (noncancerous) pituitary tumor. |
When TSH levels are out of range, your provider may order T3 and T4 levels to determine if the imbalance originates from the thyroid or pituitary gland. If TSH levels move in the opposite direction of T3 and T4, the imbalance likely originates from the thyroid. If TSH levels rise or fall alongside T3 and T4, the imbalance likely arises from the pituitary gland.
While this is not an exhaustive list, your provider may also order the following blood work to rule out underlying health conditions:
- Adrenocorticotropic hormone (ACTH)
- Chemistry (includes sodium and creatinine)
- Cholesterol and triglycerides
- Complete blood count (CBC)
- Cortisol
- Estrogen
- Gonadotropins (LH/FSH)
- Insulin-like growth factor-1 and growth hormone (serum IGF-1/GH)
- Kidney function
- Prolactin
- Testosterone
Your healthcare provider might order additional diagnostic tests such as a 24-hour urine test to check cortisol levels, an ultrasound, computed tomography scan (CT), or magnetic resonance imaging (MRI). They may also order a thyroid scan or a radioactive iodine uptake test, a noninvasive imaging test that occurs after you ingest iodine or receive it intravenously.
Your pituitary gland makes TSH to regulate thyroid hormone (T3 and T4) production. Thyroid hormones regulate energy levels, weight, temperature, heart rate, and mood. Providers measure TSH levels through a blood draw.
High or low TSH levels may indicate a problem with your thyroid or pituitary gland, including hypothyroidism and hyperthyroidism. Levels can also change with age, pregnancy, or hormonal birth control.
