Hypothyroidism

What it feels like

Tiredness that sleep does not fix, feeling cold when others do not, dry skin, hair thinning, constipation, a slowed heart rate, heavier or irregular periods, low mood and difficulty concentrating. Weight gain occurs but is usually modest — a few kilograms, much of it fluid.

None of these is specific. Most people with these symptoms have normal thyroid function, and some people with clearly abnormal results feel entirely well. The diagnosis is made on the blood test, and the symptoms tell you how much the result is costing you.

What causes it

The commonest cause where iodine intake is adequate is autoimmune thyroiditis — Hashimoto's — in which the immune system gradually destroys the gland. Anti-TPO antibodies are usually positive and confirm the cause, though they do not change the treatment.

Other causes: previous thyroid surgery or radioiodine; amiodarone and lithium; and iodine deficiency, historically important in parts of northern Pakistan and much reduced by salt iodisation. After childbirth a transient thyroiditis can cause a period of overactivity followed by underactivity, which often recovers — a reason not to commit to lifelong treatment on a postpartum result alone.

Treatment, and the parts that get missed

Treatment is levothyroxine, a synthetic form of the hormone the gland is failing to make. Full replacement is approximately 1.6 micrograms per kilogram of body weight per day, but a lower starting dose — often 25 to 50 micrograms — is used in older people, in anyone with heart disease, and when the deficiency has been present a long time.

Timing decides how much of the dose you absorb. Take it on an empty stomach, thirty to sixty minutes before breakfast, or at bedtime three hours after eating. Keep calcium, iron and antacids at least four hours away from it — this single mistake is behind a large share of the cases where the dose seems never to work.

Recheck TSH six to eight weeks after starting or changing a dose, and not sooner: TSH lags behind, and a test at three weeks will mislead you into changing a dose that was correct. Once stable, annual testing is enough.

One further caution. Thyroxine is not a weight-loss drug, and taking more than you need does not make you slimmer — it makes you at risk of atrial fibrillation and osteoporosis.

Pregnancy

Thyroxine requirement rises early in pregnancy, typically by a quarter to a third, and untreated hypothyroidism carries real risk to the pregnancy and to the child's neurological development. If you are already on thyroxine and become pregnant, do not wait for an appointment to act — contact us as soon as the test is positive, because the dose usually needs increasing straight away and TSH targets in pregnancy are tighter than outside it.

If you are planning a pregnancy, this is worth sorting out before conception rather than after.

Subclinical hypothyroidism

TSH raised, free T4 normal. Treatment is generally warranted when TSH is above 10 mIU/L, in pregnancy or when planning it, and often when TSH is persistently between the upper limit and 10 with positive antibodies or clear symptoms — with an honest agreement to stop if symptoms do not improve.

Below that, in an older person with a mildly raised TSH and no symptoms, watching is frequently the better medicine. TSH rises naturally with age, and treating a number rather than a patient does harm.