Low TSH: Causes (Subclinical Hyperthyroidism) and What to Do Next

Subclinical hyperthyroidism is a state in which TSH is low while the concentrations of free thyroxine (FT4) and free triiodothyronine (FT3) are within the reference range. When the hormones are raised, we speak of overt hyperthyroidism. The European Thyroid Association (ETA) in its 2015 guideline divides this condition into two grades: grade 1 is TSH 0.1–0.39 mIU/L (low but detectable), and grade 2 is TSH below 0.1 mIU/L (so-called suppressed TSH). The division is practical because the risk of complications rises with the depth of suppression.

In a large pooled analysis of 52,674 participants from 10 cohorts, after excluding people taking thyroid-altering medication, endogenous subclinical hyperthyroidism (TSH below 0.45 mIU/L with a normal FT4) was found in 4.2% of those studied. It is therefore a common result that many doctors ask to be repeated rather than treating at once — and rightly so, because in a significant proportion of cases the TSH lowering is transient.

Three related patterns are worth remembering. First, “T3 toxicosis”: TSH low, FT4 normal, FT3 raised — this is already overt hyperthyroidism, which is why FT3 should always be measured as well when TSH is low. Second, a low TSH together with a low FT4 suggests central (pituitary or hypothalamic) hypothyroidism or severe general illness — a completely different diagnostic path from hyperthyroidism. Third, low TSH with high hormones is classic overt hyperthyroidism, requiring prompt evaluation by an endocrinologist.

The ETA guideline lists three main causes of endogenous subclinical hyperthyroidism: Graves’ disease (autoimmune, with antibodies against the TSH receptor), autonomously functioning thyroid nodules and multinodular goitre. In the first, the thyroid is stimulated by antibodies; in the others, fragments of tissue produce hormones independently of the pituitary. Both groups can lead over time to overt hyperthyroidism, although the pace of that progression is often slow and depends, among other things, on the type of cause.

Iodine and drug treatment are usually not the first suspects, but they are worth bearing in mind. We write about iodine excess and deficiency in relation to the thyroid in a separate article, and about the importance of the right levothyroxine dose in our articles on combining T3 and T4 hormones and on how to take levothyroxine. Below we dwell longer on two causes that are the easiest to miss: too high a drug dose and biotin.

Biotin (vitamin B7) is an ingredient of many “hair, skin and nails” supplements, in doses many times higher than the body’s requirement. Many laboratory analysers use the biotin–streptavidin complex in immunoassays as an element of the measurement technique. An excess of biotin in the blood sample can disturb this system and produce results that look like hyperthyroidism (low TSH, high hormones) even though the thyroid is working normally.

The same mechanism has been described in a clinical case report: in an asymptomatic 60-year-old woman taking biotin at a very high dose (3 × 100 mg a day, for multiple sclerosis) tests showed a TSH of 0.02 mU/L and extremely high hormones, and even raised antibodies against the TSH receptor, suggesting Graves’ disease. When the tests were repeated on other platforms that do not use biotin in their reagents, the result changed — one method showed normal TSH and hormones. The case concerned a very high, medical dose; typical hair supplementation is lower but can also interfere with some tests.

In people taking levothyroxine, a low TSH is often the result of too high a dose of the drug (over-replacement). It happens after a dose increase, after weight loss, after a change of brand, as a result of changes in absorption (for example after stopping a drug that previously limited it), and when the patient takes the drug more often than prescribed. For the safety of the heart and bones, the TSH target for most people with hypothyroidism is a value within the reference range, not the lowest possible; the exception is patients treated for thyroid cancer, in whom the doctor may deliberately aim for TSH suppression.

You should not reduce your levothyroxine dose yourself after seeing a low TSH. First it is worth repeating the test (taking the tablet just before the blood draw can transiently raise FT4 — how to prepare for the test is covered in our separate article on the timing of the TSH test), measuring FT4 and FT3, and leaving the decision to change the dose to the treating doctor. People with true over-replacement usually have an FT4 in the upper part of the range or above it.

Both studies lead to a consistent conclusion: the depth of TSH suppression matters. A slightly lowered TSH (0.1–0.4 mIU/L) is associated with a small or undetectable rise in risk, whereas TSH below 0.1 mIU/L is associated with a clear one. In addition, the cardiac risk does not apply only to older people: in the pooled analysis it did not differ significantly by age. It should always be remembered, however, that these are observational data and that TSH is only a marker — people with a low TSH may differ from the rest of the population in other characteristics too.

The mechanism is well understood: an excess of thyroid hormones accelerates bone remodelling and promotes bone loss, especially in postmenopausal women. For this reason, in people with a chronically low TSH — particularly after menopause or with other risk factors — it is worth discussing a fracture risk assessment with a doctor. We write more about osteoporosis and its diagnosis in our knowledge-base entry.

The ETA guideline recommends confirming the persistence of TSH suppression on successive measurements and establishing the cause before a decision on treatment. Detailed pathways — including the place of scintigraphy — are also described in the American Thyroid Association guidelines on hyperthyroidism and thyrotoxicosis from 2016, which contain a separate chapter on subclinical hyperthyroidism.

The guideline is expert-based — it rests on observational data rather than large randomised treatment trials, and the authors themselves note this gap. Treatment, when indicated, depends on the cause: in Graves’ disease and autonomous nodules, antithyroid drugs, radioactive iodine or surgery are used, and in over-replacement with levothyroxine, adjustment of its dose. Central hypothyroidism and low T3 syndrome require entirely different management.

A low TSH is an example of a result that settles nothing by itself but always deserves further steps. Good practice is to repeat the test with FT4 and FT3, take a history for drugs, iodine and supplements (including biotin), and then match further diagnostics to the depth of suppression, age and symptoms. In many people the problem turns out to be transient or related to a drug dose; in the others, early recognition of Graves’ disease or autonomous nodules allows a decision to be made before cardiac or bone complications appear.