What is TSH?

Thyroid-stimulating hormone (TSH) is produced by the pituitary gland and stimulates the thyroid to produce the hormones T4 and T3, which help regulate metabolism and many other bodily functions.

This system works through a process known as negative feedback. When thyroid hormone levels fall, TSH levels usually rise, and vice versa.

For this reason, TSH is often the first test used to assess thyroid function in adults, provided there is no suspicion of a pituitary disorder. [1]

What Does High TSH Mean?

A high TSH level may indicate that the thyroid gland needs greater stimulation to produce sufficient thyroid hormones.

  • High TSH and low FT4: Results consistent with overt primary hypothyroidism.
  • High TSH and normal FT4: Results consistent with subclinical hypothyroidism. A mildly elevated TSH level often needs to be confirmed before any treatment decisions are made. [1], [4]

A common cause of primary hypothyroidism is Hashimoto’s thyroiditis.

However, a high TSH level alone does not confirm Hashimoto’s thyroiditis. Depending on the circumstances, thyroid antibody testing may also be recommended. [1]

What Does Low TSH Mean?

A low TSH level may be caused by elevated thyroid hormone levels.

  • Low TSH with elevated FT4 and/or T3: Results consistent with thyrotoxicosis, which may be caused by conditions such as Graves’ disease or certain forms of thyroiditis.
  • Low TSH with normal FT4 and T3: May suggest subclinical hyperthyroidism, provided the finding is confirmed and other possible explanations are ruled out. [2]

However, low FT4 with low or normal TSH may raise suspicion of central hypothyroidism, a condition associated with dysfunction of the pituitary gland or hypothalamus.

This is why TSH results should not be interpreted in isolation. [3]

What Are Normal TSH Levels?

For many adults, a reference range of approximately 0.4–4.0 mIU/L is commonly used, although this is not a universal standard.

Reference ranges can vary depending on the laboratory, testing method, age, and other factors.

Mildly elevated TSH levels require particularly careful interpretation in older adults. [4]

Different considerations apply during pregnancy. Clinical guidelines recommend using trimester-specific and population-specific TSH reference ranges when available. [5]

What Other Thyroid Tests Might Be Needed?

FT4 is particularly important when TSH levels fall outside the reference range.

When TSH is low, FT3 testing may also be needed, while anti-TPO antibodies can help investigate a possible autoimmune cause.

Not everyone needs all of these tests. The choice depends on the initial results and the individual’s clinical circumstances. [1]

What Can Affect TSH Test Results?

Pregnancy, serious or recent illness, and certain medications can affect TSH measurements or how the results are interpreted.

In addition, supplements containing high doses of biotin (vitamin B7) can interfere with certain laboratory tests, potentially producing misleading results.

This has been demonstrated in a prospective study examining different laboratory testing platforms. [1], [6]

It is important to inform both your doctor and the laboratory about any medications or supplements you are taking, rather than stopping treatment without medical advice.