What is an underactive thyroid?

Qassi Gaba · · 5 min read

An underactive thyroid means the thyroid gland is not producing enough thyroid hormone for the body's needs. The medical name is hypothyroidism, and it is one of the most common hormone conditions there is.

That's the short version. The rest of this is what the gland actually does, why it stops doing it, what the shortfall looks like in the body, and how a doctor confirms it.

What the thyroid does when it's working

The thyroid is a small gland at the front of the neck. It makes two hormones, thyroxine (T4) and triiodothyronine (T3), and releases them into the blood in response to a signal from the pituitary gland called thyroid stimulating hormone, or TSH. These hormones set the pace of metabolism in almost every tissue: how fast cells use energy, how the heart beats, how digestion moves, how body temperature is regulated. The American Thyroid Association has a plain description of what each of these hormones and TSH actually measure.

When the gland makes less T4 and T3 than the body needs, the pituitary responds by raising TSH, trying to push the gland harder. That's why TSH, not T4, is usually the first number a doctor looks at.

Why it happens

The most common cause, in countries where dietary iodine is not a problem, is Hashimoto's thyroiditis, an autoimmune condition where the immune system makes antibodies that attack the thyroid gland and gradually reduce how much hormone it can produce. It runs in families and is far more common in women than men.

It is not the only cause. An underactive thyroid can also follow surgical removal of the gland, radioactive iodine treatment for an overactive thyroid, certain medications, radiation to the neck for cancer treatment, or, in parts of the world with low dietary iodine, iodine deficiency itself. The NIDDK has a fuller list of causes and how common each one is.

Some people are born with a thyroid that never developed properly, or is missing altogether, called congenital hypothyroidism. That's why newborns are screened for it in many countries.

What it does to the body

Because thyroid hormone affects nearly every system, the list of things an underactive thyroid can cause is long and often nonspecific on its own: tiredness, weight gain, feeling cold when others don't, dry skin, hair thinning, constipation, low mood, slowed thinking, muscle aches, heavier or irregular periods. None of these on their own point clearly at the thyroid, which is part of why the condition often gets missed or blamed on something else for a while. The NHS overview lists the symptoms it's commonly associated with.

Left untreated for a long time, it can affect cholesterol levels and heart function, and in severe cases lead to a rare but serious complication called myxoedema coma. Weight change specifically is smaller than most people expect. The ATA's page on thyroid and weight is worth reading if that's the symptom that brought you here, because the relationship is real but modest, not the whole explanation for significant weight gain.

How it's tested

A doctor confirms an underactive thyroid with blood tests, not with symptoms alone, because the symptoms overlap with too many other things.

The first test is usually TSH. A high TSH suggests the pituitary is compensating for a gland that isn't producing enough hormone. If TSH is high, the next step is usually Free T4, to see whether the hormone level itself is low or whether the body is still managing to keep it in range.

If Hashimoto's is suspected, a doctor may also order a test for thyroid antibodies, most often thyroid peroxidase antibodies, which show up in most people with autoimmune thyroid disease, though not everyone. The NHS explains what a standard blood test involves if this is the first time you've had one drawn.

There's a distinction worth knowing: subclinical hypothyroidism is when TSH is mildly raised but Free T4 is still within its normal range. Whether and how that's managed is a clinical judgement that depends on the degree of TSH elevation, symptoms, and other factors, and it's exactly the kind of question that belongs with the doctor who ordered the test rather than with an article.

How it's treated

The standard treatment for an underactive thyroid is a synthetic version of T4 called levothyroxine, taken daily to replace what the gland isn't making. The ATA's guidance on thyroid hormone treatment covers how doses are chosen and adjusted, and the NHS page on levothyroxine covers how it's usually taken. Dosing is individual, based on blood tests and how someone responds, and it's reviewed periodically rather than set once and left alone.

The British Thyroid Foundation is a UK patient charity with further reading on living with the condition day to day, separate from the clinical guidelines above.

Where this touches Haloscan

If you've just been told you have an underactive thyroid, the part that tends to wear people down isn't any single appointment. It's keeping track of results and symptoms across years of them, which is what my mom did with a spreadsheet for six years before this app existed. Haloscan logs labs, symptoms and medication together and builds a two-page summary to bring to an appointment, so the pattern across visits is visible rather than reconstructed from memory each time.

Sources

Written by a software developer whose family lives with this condition, not by a clinician. Sources are linked so you can check them. It is not medical advice, and nothing here should change what you do without talking to your doctor. Who writes this · Editorial policy

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