Subclinical Hypothyroidism

What subclinical hypothyroidism means

The pituitary gland in the brain releases TSH, which tells the thyroid to make hormones (mainly T4, which converts to active T3). When the thyroid starts to struggle, the pituitary pushes harder by raising TSH. For a while, this extra push keeps T4 normal. That compensated state is subclinical hypothyroidism.

When T4 eventually falls below normal as well, the condition becomes overt hypothyroidism.

Subclinical hypothyroidism TSH levels

• Mild: TSH above the upper reference limit, up to 9.9 mIU/L. This covers about 90 percent of cases.
• Severe (grade 2): TSH 10 mIU/L or higher.

TSH naturally rises with age. A TSH of 6 to 7 can be normal in people over 70 or 80, so age-adjusted ranges are often used.

Symptoms

Most people have no symptoms. When present, they are mild and nonspecific:

• Tiredness.
• Feeling cold.
• Weight gain.
• Constipation.
• Dry skin and hair thinning.
• Low mood and poor concentration.
• Heavier or irregular periods.

These symptoms are very common in the general population, so studies have struggled to show they come from a mildly raised TSH. That is one reason treatment does not always help.

Causes

• Hashimoto's thyroiditis: the most common cause. Thyroid peroxidase (TPO) antibodies are usually positive.
• Recovery phase after illness: TSH can rise temporarily as people recover from a serious illness.
• Thyroiditis: temporary rises after viral or postpartum thyroiditis.
• Previous thyroid treatment: partial thyroid surgery, radioactive iodine, or neck radiotherapy.
• Medicines: amiodarone, lithium, some cancer drugs (immune checkpoint inhibitors, tyrosine kinase inhibitors).
• Under-replacement: too low a dose of levothyroxine, or missed doses.
• Iodine: deficiency or large excess.
• Lab factors: assay interference, and the natural variation of TSH, which is highest at night.
• Obesity: TSH tends to be slightly higher, and may fall with weight loss.

Diagnosis

1. Repeat the test in 6 to 12 weeks, including free T4. In up to half of people, TSH returns to normal.
2. TPO antibodies: positive antibodies predict a higher chance of progression.
3. Neck examination for a goiter or nodules.
4. Medication review, including supplements. Biotin can distort thyroid test results; stop it for a few days before testing.
5. Cholesterol is sometimes checked, because hypothyroidism can raise LDL.

Chance of progressing to overt hypothyroidism

About 2 to 5 percent a year progress to overt hypothyroidism. The risk is higher with TPO antibodies, TSH above 10, and in women.

Should subclinical hypothyroidism be treated?

Large trials, including one in adults over 65, found that levothyroxine did not improve tiredness, quality of life or thinking for most people with mild subclinical hypothyroidism. Several guidelines therefore recommend against routine treatment for TSH under 10.

Pregnancy and fertility

Thyroid hormone is vital for a baby's brain development, and the mother supplies it during the first trimester. Subclinical hypothyroidism has been linked with miscarriage and other complications, especially with TPO antibodies. Guidance from the American Thyroid Association advises treating pregnant women with a TSH above the pregnancy-specific range, and considering treatment at lower levels when TPO antibodies are positive. Women on levothyroxine usually need a dose increase as soon as pregnancy is confirmed.

Levothyroxine

If treatment is started, levothyroxine is taken once daily on an empty stomach, 30 to 60 minutes before breakfast, separated from calcium, iron and antacids by 4 hours. TSH is rechecked after 6 to 8 weeks. Taking too much can cause palpitations, bone thinning and atrial fibrillation, especially in older adults.

Complications

• Progression to overt hypothyroidism.
• Higher LDL cholesterol.
• A possible small increase in heart disease risk, mainly when TSH is 10 or higher and in people under 65.
• Pregnancy complications.

Frequently asked questions

What TSH level is considered subclinical hypothyroidism?

It is a TSH above the laboratory's upper limit, usually about 4 to 4.5 mIU/L, with a normal free T4. Levels up to 9.9 are considered mild, and 10 or above is considered more significant. Normal ranges are higher in older adults and lower in pregnancy.

Should subclinical hypothyroidism be treated?

Not always. For most adults with TSH below 10 and no pregnancy plans, trials show treatment does not improve symptoms, so monitoring is preferred. Treatment is usually recommended for TSH of 10 or higher, during pregnancy or when trying to conceive, and sometimes for younger people with symptoms.

Can subclinical hypothyroidism cause weight gain?

It may cause a small amount, but much less than overt hypothyroidism. Weight gain from a mildly raised TSH is usually only a few pounds, mostly fluid. Treatment with levothyroxine rarely leads to significant weight loss when TSH is only mildly raised.

Can subclinical hypothyroidism go away?

Yes. In many people, TSH returns to normal on repeat testing, especially when it was only slightly raised, TPO antibodies are negative, or the rise followed an illness. That is why doctors repeat the test before diagnosing it.

Is subclinical hypothyroidism dangerous in pregnancy?

It can increase the risk of miscarriage, preterm birth and other complications, particularly with positive thyroid antibodies. Women who are pregnant or planning pregnancy with a raised TSH should have prompt testing, and treatment with levothyroxine is often recommended.