Subclinical Hyperthyroidism: Low TSH, Normal T4

The pituitary gland makes TSH to tell the thyroid how much hormone to produce. When the thyroid produces more than the body needs, the pituitary turns TSH down. In overt hyperthyroidism TSH is suppressed and free thyroxine, free tri-iodothyronine or both are above the range. NICE Clinical Knowledge Summaries defines the subclinical form as the situation in which TSH is suppressed but FT4 and FT3 are within the reference range.

The word subclinical reflects that people often have no clear symptoms. When symptoms do occur they are those of an overactive thyroid. The NHS lists nervousness, anxiety and irritability, mood swings, difficulty sleeping, persistent tiredness, sensitivity to heat, a fast or irregular heartbeat, trembling and weight loss.

Because the definition rests on numbers rather than symptoms, the result is often a surprise found on a test done for another reason.

A low TSH is not the same as an overactive thyroid. Lab Tests Online UK explains that a low TSH can indicate an overactive thyroid or damage to the pituitary gland that prevents it producing TSH, and that it can also occur in people with an underactive thyroid who are receiving too much thyroid hormone medication.

NICE Clinical Knowledge Summaries advises that a TSH level is checked first, and if it is suppressed FT4 and FT3 are measured in the same sample. If subclinical hyperthyroidism is suspected, thyroid function tests are rechecked 3 months after the initial result, to exclude other causes of a transiently suppressed TSH. Other bloods such as inflammatory markers and thyroid autoantibodies may be considered if clinically indicated.

Lab Tests Online UK says that in people with an overactive thyroid more regular TSH monitoring may be required, up to every 6 weeks in the early stages of treatment, and that TSH should not routinely be measured more often than every 6 weeks. It also advises waiting at least 6 weeks after a dose change before rechecking TSH, so that the new dose has had its full effect.

The point of the delay is to separate a stable low TSH from a one-off. Many low results are not seen again on repeat testing.

NICE CKS advises arranging endocrinology referral for persistent subclinical hyperthyroidism if TSH levels are lower than 0.1 mU/L at least 3 months apart and there is evidence of thyroid disease. It also advises urgent endocrinology referral if a pituitary or hypothalamic disorder is suspected, and pre-conception counselling and specialist referral for women who are planning a pregnancy or are pregnant with current or previous overt or subclinical hyperthyroidism.

The CKS summary lists atrial fibrillation and heart failure among the complications of hyperthyroidism, which is why a TSH that stays suppressed is taken seriously. Beta-blockers may be considered for adrenergic symptoms such as palpitations and tremor.

The sources read here do not set out a single treatment rule for subclinical disease. Decisions about antithyroid medicine, radioiodine or surgery are specialist ones, based on the cause, the TSH level, age, symptoms and other risks.

If you take levothyroxine, do not adjust the dose on your own. The dose is reviewed by the prescriber, who will have the result and the context. If you are pregnant or planning pregnancy, tell the clinician at once, because a different reference range and pathway applies.

• Mention any recent illness, pregnancy and any new medicine, for example lithium or amiodarone, which Lab Tests Online UK says may interfere with results.

• Report palpitations, weight loss, tremor, heat intolerance or anxiety that are new.

• Keep the repeat test date, since the 3-month recheck is the main way of deciding what the result means.

• Ask whether thyroid antibodies or other tests are planned if the TSH stays suppressed.