Hypothyroidism is a condition in which the thyroid gland produces too little of the hormones thyroxine and triiodothyronine. Metabolism slows down, and the heart, skin, gut, mood and memory suffer.
It is one of the most common endocrine disorders, especially in women and with age. The diagnosis is made from blood tests — primarily TSH and free T4.
Causes
The most common cause in countries with adequate iodine intake is autoimmune thyroiditis (Hashimoto's disease), in which the immune system gradually destroys the gland's tissue. Other causes: iodine deficiency, the aftermath of treating hyperthyroidism (radioiodine, surgery), neck irradiation, certain medications (amiodarone, lithium) and congenital disorders.
Symptoms
The manifestations develop slowly and are non-specific: fatigue and drowsiness, feeling cold, dry skin, puffiness, weight gain, constipation, a slowed pulse, poorer memory and low mood, and in women — menstrual irregularities. Many complaints are non-specific, so the diagnosis is confirmed by tests, not by well-being alone.
Diagnosis
The key test is TSH: in primary hypothyroidism it is elevated, because the pituitary tries to "spur on" the gland. Free T4 is looked at at the same time. In overt hypothyroidism TSH is high and free T4 is low; in subclinical hypothyroidism TSH is elevated but free T4 is still normal. Anti-TPO antibodies help identify an autoimmune cause.
Treatment
The basis of treatment is replacement therapy with levothyroxine (synthetic T4) once a day on an empty stomach. The dose is chosen by body weight and monitored by TSH 6–8 weeks after each change. With a correctly chosen dose, well-being and tests normalize; with an autoimmune cause, treatment is usually lifelong.
Reference ranges
Approximate criteria from blood tests (the TSH thresholds depend on the laboratory and age):
| TSH, reference range | 0.4 – 4.0 mIU/L |
| Subclinical hypothyroidism | TSH 4–10 mIU/L, free T4 normal |
| Overt hypothyroidism | TSH >10 mIU/L or free T4 reduced |
Calculate using this marker
Calculators where Hypothyroidism is used directly:
Frequently asked questions
What is subclinical hypothyroidism?
It is a mild form, when TSH is already elevated but free T4 is still within the normal range. There may be no symptoms. The approach (observation or treatment) is decided by the TSH level, age, symptoms, antibodies and pregnancy planning.
Can hypothyroidism be cured for good?
If the cause is reversible (for example, subacute thyroiditis or medications), function can recover. In autoimmune thyroiditis the gland does not recover, and levothyroxine is usually lifelong, but it fully compensates for the hormone deficiency.