Subclinical hypothyroidism: symptoms, treatment and what a borderline TSH means

The BTF says most people will have subclinical hypothyroidism detected through tests for unrelated symptoms. Even when someone has symptoms of an underactive thyroid it is difficult to be certain that they are related to the thyroid, because the symptoms are vague and could be due to many other conditions. For example, fatigue or weight gain are more likely due to lifestyle or other common conditions.

The BTF lists the most common symptoms of an underactive thyroid, which in subclinical disease are expected to be mild or absent. The NHS adds that symptoms of an underactive thyroid may be mild and not easy to notice, usually develop slowly and get worse over time.

• Fatigue and tiredness

• Increased awareness of the cold

• Dry skin and dry, thinning hair

• Hoarse or croaky voice

• Slight weight gain and constipation

• Muscle aches, low mood and difficulty concentrating

• Heavier and longer periods, fertility problems

The BTF says doctors take into account factors that may suggest underlying thyroid disease, such as symptoms of hypothyroidism, previous thyroid surgery or radioiodine treatment, or raised thyroid antibodies. If you are an adult under 65 with symptoms of hypothyroidism and your TSH is raised on two separate occasions at least three months apart (but lower than 10 mU/l), your doctor may consider a six-month trial of levothyroxine.

TSH is re-measured within 12 weeks of starting, and the dose may be increased if it stays raised. If symptoms persist despite TSH returning to the reference range, the trial may be stopped and other reasons for the symptoms explored while monitoring continues.

Treatment is not routine for everyone. For people over 65 an improvement in symptoms is less likely and there is greater potential for harm from suppressing TSH, such as strain on the heart or an increased risk of brittle bones. For people with increased body weight, the BTF says trials have shown no benefit of levothyroxine in reducing body weight, so treatment is not recommended purely to reduce weight.

The pituitary gland releases TSH to prompt the thyroid to make thyroxine (T4). In subclinical hypothyroidism the TSH rises first while the T4 is still in the normal range. The BTF describes it as also known as mild or borderline hypothyroidism, a common condition that occurs more frequently in women and as we age.

Laboratories use slightly different reference ranges, so check the range printed on your own report. A single borderline result is rarely enough to act on, which is why repeat tests are usual.

The BTF says it can be caused by autoimmune thyroid disease (Hashimoto's thyroiditis, shown by TPO antibodies), which means a higher risk of developing full hypothyroidism in the future. Other causes it lists are radioactive iodine treatment or surgery for hyperthyroidism or thyroid cancer, too high doses of antithyroid drugs, certain medicines (lithium, amiodarone and immune checkpoint inhibitors such as pembrolizumab and nivolumab), excess iodine from some health foods such as kelp, radiation for head and neck cancers, older age and increased body weight.

The NHS says that in the UK the main cause of a full underactive thyroid is Hashimoto's disease, most common in women and usually starting between 30 and 50.

The NHS says a GP may arrange blood tests to check how much TSH and T4 is in your blood. The BTF says that if you have subclinical hypothyroidism you will usually be tested for TPO antibodies to establish whether the underlying cause is autoimmune.

Monitoring depends on risk. The BTF says your doctor will usually monitor thyroid function annually if you have positive antibodies, previous thyroid surgery or radioactive iodine treatment, or have come off a levothyroxine trial. Where there is no suggestion of thyroid disease, thyroid function is normally measured every two to three years.

The BTF says the relationship between thyroid function and body weight is complex. An underactive thyroid leads to slight weight gain, typically 3 to 4 kg, but weight gain can also lead to a slightly raised TSH with normal T4. It adds that weight reduction by other means, such as diet and exercise, has been shown to normalise or reduce TSH.

The BTF lists excess iodine from some health foods such as kelp as a possible cause. The NHS says that not getting enough iodine in the diet is a rare cause in the UK. Do not take iodine supplements or kelp unless a doctor advises it, as both too little and too much can affect thyroid tests.

The BTF says that if you are trying for a baby, or are pregnant, you would normally be treated with levothyroxine if your TSH is greater than the pregnancy-specific reference range, or greater than 4 mU/L if there are no pregnancy ranges available locally. If your thyroid peroxidase antibodies are raised, it notes that American Thyroid Association guidelines recommend treatment if TSH is greater than 2.5 mU/l, though the benefit of treatment in that context is less clear.

The NHS says if you have an underactive thyroid and are pregnant or planning to get pregnant, tell your GP, as you need further tests and your levothyroxine dose is likely to change.

See a GP if you think you might have an underactive thyroid, as the NHS advises, particularly if tiredness, cold intolerance, constipation or low mood are persistent. A borderline result alone is not usually urgent.

The NHS says a severe untreated underactive thyroid can very rarely lead to myxoedema coma, an emergency that needs hospital treatment and is more common in older people. Call 999 for extreme drowsiness, confusion or collapse in someone with known thyroid disease.

Thyroid function tests are done in NHS primary care, and a GP usually manages subclinical hypothyroidism, referring to endocrinology where there is doubt or complexity. Private endocrinology and private blood tests are available for people who want an earlier appointment or extra testing.

Whichever route you use, ask for results and the reference range to be shared with your GP, because treatment decisions depend on repeat results over time rather than a single test.