Hypothyroidism

What hypothyroidism is

The thyroid is a small butterfly-shaped gland at the front of the neck. The hormones it makes, thyroxine (T4) and triiodothyronine (T3), control the pace of metabolism: how fast the heart beats, how much heat the body makes, how fast the bowel moves and how much energy cells have. Their production is directed by the pituitary gland, which releases TSH (thyroid-stimulating hormone). When the thyroid makes too little hormone, the pituitary responds by raising TSH in an attempt to stimulate it, and this is why TSH is the main test for diagnosis.

When hormone levels fall, every system slows down, and this slowing explains most of the symptoms. In most cases the disease develops over months or years, so the body gets used to it and the patient attributes the fatigue to age, stress or work. Hypothyroidism is more common in women and with advancing age.

Doctors distinguish overt hypothyroidism (high TSH and low free T4, or fT4) from subclinical hypothyroidism (high TSH with normal fT4). The first almost always needs treatment; the second is assessed individually, as explained below. Very rarely the problem is not in the thyroid but in the pituitary (central hypothyroidism); then TSH may be low or normal while fT4 is low, so the results must be interpreted by a doctor.

Causes

In countries where iodine deficiency is not a major problem, the most common cause is Hashimoto’s autoimmune thyroiditis. In this disease the immune system produces antibodies (especially anti-TPO) against the thyroid and over time gradually damages the gland. It runs in families and often goes with other autoimmune diseases, such as type 1 diabetes or celiac disease.

Other causes are:

• removal of the thyroid or part of it by surgery, or treatment with radioactive iodine or radiation to the neck (for example for hyperthyroidism or cancer);
• certain medicines, such as amiodarone, lithium, some cancer drugs (such as immunotherapies), and iodine in large amounts;
• inflammation of the thyroid after childbirth or after a viral infection (often temporary);
• iodine deficiency or excess;
• being born without a thyroid, or damage to the pituitary.

Symptoms

The symptoms are varied and nonspecific, meaning many other conditions can cause them too, so only a blood test confirms the diagnosis. The most common are:

• fatigue, sleepiness and a lasting lack of energy;
• sensitivity to cold;
• mild weight gain without any change in diet;
• dry skin, hair loss, brittle nails, a hoarse voice;
• constipation;
• menstrual changes (heavy or irregular periods) and difficulty getting pregnant;
• poor concentration, forgetfulness and low mood;
• a slow pulse and raised cholesterol (see dyslipidemia).

Weight rises only a little (usually a few kilograms, partly from retained fluid); hypothyroidism does not explain large weight gain. Conversely, levothyroxine should never be used to lose weight: in a person with a normal thyroid it does not help and is dangerous.

Diagnosis

The diagnosis is made with a blood test. TSH is the first and most sensitive test; when it is raised, fT4 is checked as well. If an autoimmune cause is suspected, anti-TPO is also measured: a high value points to Hashimoto’s and predicts that the disease may progress. T3 does not need to be measured for the diagnosis.

Thyroid ultrasound helps to assess the structure of the gland: in Hashimoto’s thyroiditis the thyroid often looks uneven and less dense, and nodules may be found at the same time. It is not needed for the diagnosis but adds to the complete picture.

A few tips on testing: take your levothyroxine after the blood draw on the day of the check; if you take biotin (vitamin B7) in high doses, it can change the results of some hormone tests, so tell your doctor or stop it for a few days before the test, as advised. A mildly raised TSH during or after an acute illness may be temporary; the doctor repeats the test before starting treatment.

Treatment

The standard treatment is levothyroxine, a synthetic form of the hormone T4, identical to what the body makes. It is taken once a day by mouth, usually for life when the cause is permanent (for example Hashimoto’s or after removal of the thyroid). With proper treatment people live a completely normal life.

How to take it. Levothyroxine is absorbed best on an empty stomach: take it 30–60 minutes before breakfast with water, or at night, three hours after the last meal. Iron, calcium, antacids and some supplements can reduce its absorption, so separate them from it by at least 4 hours. Always take the same brand, because changing brands can slightly change the effective dose.

The dose. It is set by the doctor according to weight, age, cause and heart disease. In older people and those with coronary disease, treatment begins at low doses and rises gradually. After starting or any dose change, TSH is checked after 6–8 weeks and the dose adjusted until the value is in range; after that, checks are usually every 6 months to a year, or sooner when the doctor considers it reasonable: in pregnancy, when weight changes or when other medicines are started. Too high a dose is as harmful as too low a dose, because it can cause palpitations, atrial fibrillation, insomnia and bone loss; so do not change it yourself.

If you forget a tablet, take it as soon as you remember; do not make a habit of doubling doses without asking your doctor.

Subclinical hypothyroidism. When TSH is mildly raised and fT4 is normal, the doctor usually repeats the test after 2–3 months and decides whether to treat. Treatment is considered more often when TSH is high (for example above 10 mIU/L), when there are symptoms, positive anti-TPO, pregnancy or planning a pregnancy, or cardiovascular risk; in older people with a mildly raised TSH, follow-up without treatment is often more reasonable.

Pregnancy and special situations

In pregnancy the need for thyroid hormone rises, because the mother’s thyroid also covers the baby’s early brain development. Women who take levothyroxine should tell their endocrinologist as soon as pregnancy is confirmed (or when planning), because the dose usually goes up and TSH is checked more often; untreated hypothyroidism raises the risk of complications such as miscarriage, preterm birth and preeclampsia. When planning a pregnancy, it is wise to have a TSH test beforehand.

Severe untreated hypothyroidism can lead to myxedema coma, a rare but dangerous emergency, more common in older people, triggered by cold, infections or stopping treatment; it goes with severe drowsiness, confusion, low temperature and slow breathing, and needs hospital care.

When to see a doctor

• when you have unexplained fatigue, sensitivity to cold, constipation and dry skin;
• when a test shows TSH outside the range;
• when you are planning a pregnancy and have thyroid disease or a family history;
• when you have started treatment and still have symptoms, or develop a fast heartbeat, tremor or insomnia, which may point to too high a dose;
• when you have noticed a swelling in the neck (see thyroid nodules).

Bring your latest tests and your list of medicines. This condition is managed by endocrinology.