TSH test explained

TSH (thyroid-stimulating hormone, thyrotropin) is made by the pituitary gland and works like a thermostat: when thyroid hormone in the blood falls, the pituitary releases more TSH to push the thyroid to make more T4; when there is too much hormone, TSH is suppressed. Because of this feedback loop, TSH often shifts before the thyroid hormones themselves change, which is why it is considered the best first thyroid test. If TSH is normal, the thyroid is most likely working well; if it is not, the lab usually adds free T4 (FT4), and free T3 (FT3) when TSH is low. Interpreting TSH always depends on FT4: a high TSH with a low FT4 means primary hypothyroidism (underactive thyroid), a low TSH with a high FT4 means hyperthyroidism (overactive thyroid), and a mildly raised TSH with a normal FT4 is called subclinical hypothyroidism. Pregnancy, certain medicines, serious illness and biotin supplements can all affect the TSH result.

• Signs of an underactive thyroid: tiredness, weight gain, feeling cold, dry skin, hair thinning, constipation, irregular periods, low mood
• Signs of an overactive thyroid: weight loss despite a normal appetite, a fast heartbeat, nervousness, insomnia, tremor, sweating, frequent bowel movements, an enlarged thyroid
• When planning a pregnancy and during pregnancy
• To monitor treatment of a thyroid condition — the treatment goal is usually a TSH within the reference range
• When other results (cholesterol, prolactin) change unexpectedly and the thyroid may be the cause

A raised TSH usually means the thyroid is producing too little hormone — primary hypothyroidism, most often caused by Hashimoto's thyroiditis, an autoimmune inflammation of the thyroid. If free T4 is still within its normal range, this is called subclinical hypothyroidism. Recovering from a serious illness and certain medicines can also raise TSH temporarily. Typical causes include an insufficient dose of thyroid hormone replacement, medicines such as amiodarone or lithium, and excess iodine. Typical symptoms are tiredness, weight gain, feeling cold, joint and muscle aches, dry skin, hair thinning, irregular periods, infertility, low mood and constipation. Next steps usually assessed alongside a raised TSH: free T4 (FT4), which shows whether the hypothyroidism is subclinical or already overt; thyroid antibodies (anti-TPO) to check for an autoimmune cause; any medicines and supplements taken, including biotin; and when to repeat the test, since subclinical hypothyroidism is generally monitored over time.

A low TSH usually points to an overactive thyroid (hyperthyroidism) — Graves' disease, toxic nodular goitre — or too high a dose of thyroid hormone replacement. Less often the cause is a pituitary disorder, where both TSH and FT4 are low. Certain medicines — glucocorticoids, dopamine, opioids — can also suppress TSH, and TSH can be physiologically lower in early pregnancy. Typical symptoms are weight loss despite a normal appetite, a fast heartbeat, nervousness, insomnia, tremor, sweating, frequent bowel movements and an enlarged thyroid (goitre). Next steps usually assessed alongside a low TSH: free T4 and T3, which distinguish hyperthyroidism from a pituitary cause; TSH-receptor antibodies (TRAb) to check for Graves' disease; medicines, iodine sources and supplements taken; and whether pregnancy is a factor, since TSH is assessed differently in early pregnancy.

TSH 4-10 mU/l (subclinical hypothyroidism): This is just above the British Thyroid Foundation's typical adult range of 0.4-4.0 mU/l. If free T4 is normal at the same time, it is called subclinical hypothyroidism — a mild form that a doctor usually monitors, testing thyroid antibodies and repeating the test, rather than treating immediately. The BTF notes that subclinical hypothyroidism can progress to overt hypothyroidism over time, so it recommends also testing thyroid antibodies (anti-TPO) to gauge that likelihood. A single TSH result on its own proves little: it can be temporarily affected by serious illness, certain medicines (for example amiodarone or lithium) or biotin supplements, so a doctor weighs it together with symptoms, the FT4 value and, if needed, a repeat test after some weeks or months.

TSH above 10 mU/l: This is clearly raised. According to the British Thyroid Foundation, even when free T4 is still normal at this level, some people — especially women planning a pregnancy — may benefit from treatment; this is the one specific numeric threshold the BTF names for considering treatment of subclinical hypothyroidism. If FT4 is also low at this point, it is classed as overt (clinical) hypothyroidism, which the BTF says usually needs treatment — so the FT4 result is essential to tell the two situations apart. TSH can also rise temporarily while recovering from serious illness or on certain medicines, so a very high result is usually rechecked before any decision is made.

TSH below 0.4 mU/l: This is below the British Thyroid Foundation's typical range. It usually points to an overactive thyroid, especially if free T4 or T3 is also raised; less often the cause is a pituitary disorder, or a deliberately low TSH maintained as part of thyroid cancer treatment. A result this low is uncommon and is usually confirmed with a repeat test before any decisions are made. When low TSH occurs together with raised FT4, this indicates hyperthyroidism (an overactive thyroid), which the BTF says needs treatment — the most common causes being Graves' disease or a toxic nodular goitre. Less often, low TSH together with low FT4 points not to the thyroid but to a pituitary problem (secondary hypothyroidism), or is a temporary response to serious illness unrelated to the thyroid — so the FT4 (and sometimes FT3) result is essential to tell these causes apart, since a low TSH alone does not. It is also worth noting that in people treated for thyroid cancer, doctors deliberately keep TSH below the reference range (usually 0.1-0.5 mU/l), so in that context a low TSH can be the intended treatment target rather than a sign of a problem — interpretation always depends on the individual's circumstances and treatment history.

In Lithuania, a family doctor can order a TSH test when clinically indicated, sometimes free of charge. Paying privately, a TSH test at a Lithuanian laboratory such as Medicina practica costs from around €12 (September 2026 pricing).