Hypothyroidism: An Underactive Thyroid Explained

What the thyroid does

The thyroid is a butterfly-shaped gland sitting across the windpipe at the base of the neck. It makes two hormones, thyroxine (T4) and the more active triiodothyronine (T3), which set the speed at which every cell works — heart rate, body temperature, how quickly food becomes energy, how fast the bowel moves, how the brain feels.

It does not decide its own output. The pituitary gland at the base of the brain monitors the level in the blood and sends an instruction called thyroid stimulating hormone, or tshTSHTSH is the pituitary hormone that tells the thyroid how hard to work, and it is the first blood test used to check thyroid function. It reads in the opposite direction to intuition — a high TSH means an underactive thyroid.Read the full definition. When thyroid hormone is low, the pituitary shouts louder and TSH rises. That relationship is the key to reading a thyroid report, and it is why a high TSH means an underactive thyroid rather than an overactive one — a point that confuses almost everyone the first time. When output falls the body slows measurably: the pulse drops, the bowel becomes sluggish, the skin dries, the voice deepens, cholesterol rises, and thinking and mood flatten. It comes on so gradually, often over a year or more, that people attribute the whole picture to age, stress or young children.

What causes it

• Autoimmune thyroiditis, or Hashimoto's disease, in which the immune system gradually damages the gland. This is the commonest cause wherever iodine intake is adequate, is far more frequent in women, and is confirmed by finding thyroid peroxidase antibodiesAntibodyAn antibody is a protein made by the immune system that locks onto one specific target on a germ and marks it for destruction. Antibodies are what give a person lasting protection after an infection or a vaccination.Read the full definition in the blood.
• Iodine deficiency. The thyroid cannot make its hormones without iodine. Pakistan has a long history of iodine-deficiency goitre, particularly in the northern mountain districts, and universal salt iodisation was introduced for exactly this reason. Household use of iodised salt has improved but is still not universal.
• Treatment of an overactive thyroid, by surgery or radioactive iodine. Hypothyroidism afterwards is expected rather than a complication.
• Medicines, most notably amiodarone for heart rhythm problems and lithium for bipolar disorder. Anyone on either needs periodic thyroid tests.
• After childbirth. Postpartum thyroiditis affects a small percentage of women in the year after delivery, often with a brief overactive phase followed by an underactive one. It frequently recovers, but it needs monitoring.
• Pituitary disease, which is rare. Here the problem is the instruction rather than the gland, so TSH is low or normal while free T4 is low — the reverse of the usual pattern.

Reading your thyroid report

Two practical points about testing. TSH varies through the day and is higher in the early morning, so a single borderline result is repeated rather than acted on, usually after two to three months. And biotin, sold widely for hair and nails and present in many multivitamins, interferes with the laboratory method used for thyroid tests and can produce a strikingly abnormal result in a healthy person — stop it for a couple of days beforehand and tell the laboratory you were taking it.

Subclinical hypothyroidism: treat or watch?

A raised TSH with a normal free T4 is common, particularly in older people, and does not automatically require a tablet. Broadly, treatment is offered when the TSH is above 10 mIU/L on two occasions a few months apart, since that group is most likely to progress and to benefit. Between the top of the normal range and 10, the decision depends on whether you have symptoms that fit, whether thyroid antibodies are present, your age, and whether you are pregnant or trying to conceive — in which case treatment is offered at a much lower threshold. Where treatment is not started, the sensible plan is a repeat test in a few months and then annually, because some people progress and some return to normal on their own. In older people a mildly raised TSH may simply be part of ageing, and treating it can do more harm than good.

Levothyroxine: how to take it so it works

Levothyroxine is a synthetic copy of the body's own T4 — less a drug than a replacement for something missing, which is why it has no side effects when the dose is right; the problems come only from too much or too little. Your prescriber sets the dose from your age, weight, heart and how long the thyroid has been underactive, and adjusts it from repeat blood tests. Do not adjust it yourself, and do not use someone else's prescription as a guide. How you take it matters more than with most medicines, because its absorption is easily blocked, and getting that wrong is the commonest reason a dose appears not to be working.

• Take it once a day at the same time, ideally 30 to 60 minutes before breakfast with water only. If mornings are impossible, bedtime at least three hours after food works comparably well.
• Keep it four hours apart from calcium, iron, magnesium and multivitamins, and from antacids containing aluminium or magnesium. These bind it in the gut and it never reaches the blood.
• Separate it from tea, coffee and milk, which also reduce absorption. Water only.
• Stay on the same brand where you can. Preparations are not always precisely interchangeable, and unexplained TSH changes sometimes trace back to a pharmacy substitution.
• Tell your doctor about new medicines, particularly acid-reducing tablets, iron, calcium and some anti-epileptic and cholesterol drugs, which change how much you need.
• If you miss a dose, take it later the same day; if you only remember the next day, take that day's dose as usual and do not double up. The hormone stays in the body for around a week, so one missed dose makes very little difference.
• Have the blood test six to eight weeks after any dose change, then every six to twelve months once stable. Testing sooner gives a number that has not settled.

Pregnancy

This is where an underactive thyroid matters most and is most easily missed. The developing baby depends entirely on the mother's thyroid hormone through the first trimester, while its own gland is forming, and untreated hypothyroidism in pregnancy is associated with miscarriage, pre-eclampsia, premature birth, low birthweight and impaired brain development in the child.

Requirements rise early — the dose commonly needs to increase by roughly a quarter to a half within the first weeks — so anyone on levothyroxine should contact their doctor as soon as a pregnancy test is positive rather than waiting for the first antenatal visit. TSH targets in pregnancy are lower than outside it and monitoring is more frequent. If you are planning a pregnancy, ask for your thyroid to be checked and optimised beforehand; it is the single most useful preconception thyroid step there is.

What treatment will and will not fix

Correcting an underactive thyroid reliably improves cold intolerance, constipation, dry skin, hoarseness, heavy periods, a raised ldl cholesterolLDL cholesterolLDL cholesterol is the particle that carries cholesterol from the liver out to the tissues and deposits it in artery walls along the way. It is the number that treatment aims at, and lowering it reliably lowers the risk of heart attack and stroke.Read the full definition and the mental fog. Energy improves too, more slowly, and it can take three to six months before you feel fully yourself.

What it does not deliver is dramatic weight loss. Untreated hypothyroidism typically accounts for a few kilograms, much of it retained fluid, and correcting it returns that much and no more. This is worth saying plainly because thyroid hormone is sometimes taken, or even prescribed, for weight loss in people with normal thyroid function. It does not work, and at doses high enough to affect weight it causes irregular heart rhythms and accelerates bone loss towards osteoporosisOsteoporosisOsteoporosis is loss of bone density that leaves bones fragile enough to break from a minor fall or even a cough. It causes no symptoms until a fracture happens, which is what makes it dangerous.Read the full definition. If tiredness and weight gain persist despite a normal tshTSHTSH is the pituitary hormone that tells the thyroid how hard to work, and it is the first blood test used to check thyroid function. It reads in the opposite direction to intuition — a high TSH means an underactive thyroid.Read the full definition, the answer lies elsewhere — anaemiaAnaemiaAnaemia means the blood carries less haemoglobin than it should, so less oxygen reaches the tissues. It usually shows itself as tiredness, breathlessness on exertion and pallor, and in Pakistan the commonest cause by a wide margin is a shortage of iron.Read the full definition, vitamin dVitamin DVitamin D lets the body absorb calcium and is essential for bone strength. It is made in the skin on exposure to sunlight, and deficiency is very common — including in sunny countries.Read the full definition deficiency, sleep apnoea, depression, poor sleep or diabetesDiabetesDiabetes is a condition in which blood glucose stays too high, either because the body makes too little insulin or because it has stopped responding to it. Pakistan has one of the highest rates in the world, and a large share of people with it do not yet know.Read the full definition are all more likely and all worth a look.

One diet note. No food treats hypothyroidism and none needs avoiding beyond common sense. Cabbage, cauliflower and soy contain compounds that theoretically interfere with iodine use, but you would have to eat implausible quantities for it to matter when iodine intake is adequate. Do use iodised salt. Do not take kelp, seaweed tablets or high-dose iodine supplements: excess iodine can worsen thyroid disease rather than fix it.

When to see a doctor

Ask for a thyroid test if tiredness, weight gain, cold intolerance, constipation, hair thinning or low mood have persisted for weeks, if your periods have become much heavier, if you have a swelling at the front of the neck, if you have another autoimmune condition, or if you are planning a pregnancy. A tshTSHTSH is the pituitary hormone that tells the thyroid how hard to work, and it is the first blood test used to check thyroid function. It reads in the opposite direction to intuition — a high TSH means an underactive thyroid.Read the full definition with a free T4 answers the question cheaply, and it is often included in a general screeningScreeningScreening is testing people who feel well, to find a condition earlier than symptoms would. It is only worth doing when finding the condition early genuinely changes what happens next.Read the full definition panel already.

If you are already on treatment, see your doctor when a dose changes and six to eight weeks after, if symptoms return, if you become pregnant, if you start any new regular medicine, and once a year otherwise. Get urgent help for chest pain, palpitations or a racing pulse after a dose increase, and for severe drowsiness or confusion in anyone with a known underactive thyroid.