High TSH: what it means

What TSH measures

TSH (thyroid stimulating hormone) is made by the pituitary gland at the base of the brain. It tells the thyroid gland in your neck how much thyroid hormone to make. The system works like a thermostat: when thyroid hormone is low, TSH goes up to push the thyroid harder. That is why a high TSH points to an underactive thyroid (hypothyroidism). Free T4 is the main hormone the thyroid releases, and checking it alongside TSH shows how much the thyroid is struggling. Some labs report free T4 in pmol/L; multiply ng/dL by 12.9 to convert.

Common causes

The most common cause of an underactive thyroid in most countries is Hashimoto's thyroiditis, an autoimmune condition where the immune system slowly damages the thyroid. Iodine deficiency is a cause in some regions. A raised TSH can also be temporary, so a single result is rarely enough to decide on treatment. Women, people over 60 and those with type 1 diabetes or a family history of thyroid disease are more likely to have an underactive thyroid.

• Hashimoto's thyroiditis (often with positive anti-TPO antibodies)
• Recovery after a viral illness or thyroiditis
• Previous thyroid surgery or radioactive iodine treatment
• Medicines such as lithium or amiodarone
• Taking thyroid tablets irregularly or with food or calcium

Reading TSH with free T4

When TSH is high and free T4 is low (under 0.8 ng/dL), this fits overt hypothyroidism, which is usually treated. When TSH is high but free T4 is normal, it is called subclinical hypothyroidism. Many people with this have no symptoms, and some return to normal on a repeat test. Symptoms of an underactive thyroid can include tiredness, feeling cold, weight gain, constipation, dry skin, low mood and heavy periods. These are common and have many other causes, so blood tests are needed to link them to the thyroid.

What usually happens next

If your TSH is mildly raised, your doctor will often repeat TSH and free T4 in 6 to 12 weeks, sometimes with an anti-TPO antibody test. If hypothyroidism is confirmed, your doctor may suggest a daily thyroid hormone tablet (levothyroxine) and recheck TSH after about 6 weeks to adjust the dose. Tell your doctor if you are pregnant or planning pregnancy, because thyroid targets are stricter then. Biotin supplements can upset some thyroid tests, so mention them; your doctor may suggest pausing them for a couple of days before the test.

Questions to ask your doctor

• Is my free T4 normal or low, and what does that mean for me?
• Should I repeat the test before deciding on treatment?
• Would an anti-TPO antibody test help?
• Could any of my medicines or supplements affect my result?
• If I need a thyroid tablet, how and when should I take it?