Quick version
Hypothyroidism (underactive thyroid) means that the body produces too little thyroid hormone, which lowers the metabolism and slows down the body's processes. Because the symptoms – such as extreme fatigue, feeling cold, weight gain and brain fog – develop insidiously and resemble stress or vitamin deficiency, it is easy to miss the cause.
The most common cause is the autoimmune disease Hashimoto's thyroiditis. The diagnosis is easily made via blood tests that measure the hormones TSH and free T4, and the condition is effectively treated with the drug levothyroxine. Testing the thyroid is an important step in ruling out or treating a common but hidden cause of reduced quality of life and work capacity.
Do you feel tired even though you are sleeping, frozen when others think it is just right, or notice that your weight is creeping up without a clear explanation? Then you are far from alone. Many people go for a long time with diffuse problems without understanding that the thyroid gland may be part of the explanation. Hypothyroidism, i.e. underactive thyroid, often develops slowly and the symptoms can easily be confused with stress, aging, iron deficiency or fatigue. Therefore, it is important to know both the signals from the body and which blood tests can actually provide answers.
Underactive thyroid – how to know if you have hypothyroidism
Hypotyreos means that the thyroid gland produces too little thyroid hormone. The thyroid gland is located at the front of the neck and primarily produces the hormones T4 (thyroxine) and T3 (triiodothyronine), which help the body regulate metabolism, temperature, heart rate, energy metabolism and several other important functions. When levels become too low, the body's processes slow down.
To understand test results, it is good to know about TSH. It is a hormone from the pituitary gland that acts as the body's "control signal" to the thyroid gland. If the thyroid gland produces too little hormone, TSH often rises, because the body tries to pressure the thyroid gland to work more. In typical primary hypothyroidism, you often see high TSH and low free T4.
The most common cause in adults is autoimmune thyroid disease, often called Hashimoto's thyroiditis. This means that the immune system mistakenly attacks the thyroid gland, which over time can reduce hormone production. Other causes may include previous thyroid surgery, radioiodine treatment, certain medications, or inflammation of the thyroid gland.
There is also something called subclinical hypothyroidism. In this case, TSH is elevated but free T4 is still within the reference range. Some have symptoms, others do not. It is not always obvious that the condition should be treated immediately, but it needs to be assessed medically based on symptoms, test results, antibodies, age, and whether you are pregnant or planning a pregnancy.
What symptoms may indicate hypothyroidism?
The symptoms of hypothyroidism often come on insidiously. That is why they can be difficult to detect in time. Many people first think they are “just tired”, but when you look at the whole picture, the pattern becomes clearer.
Common symptoms are:
pronounced fatigue or lack of energy
feeling cold
weight gain or difficulty maintaining a stable weight
dry skin
hair loss or poor hair quality
constipation
depressed mood or slower thinking
memory problems or concentration problems
swelling in the face or body
muscle pain, stiffness or joint pain
slow pulse
hoarseness
heavy or irregular menstruation
reduced fertility or difficulty getting pregnant.
It is common to feel unusually sluggish for several months, freeze more than normal and notice that the skin has become drier. The symptoms can also manifest as depression, brain fog or constipation, which means that many people are surprised when a blood test shows that the thyroid is the cause. Hypothyroidism does not always give a completely "typical" picture from the start.
It is also important to say that the symptoms are not unique to hypothyroidism. Similar symptoms can be seen in, for example, iron deficiency, B12 deficiency, depression, lack of sleep, chronic stress or menopause. Therefore, it is not possible to make a diagnosis based on symptoms alone. Blood tests are needed.
How is hypothyroidism diagnosed?
To diagnose hypothyroidism, blood tests and a comprehensive medical assessment are performed. The most important tests are usually TSH and free T4. In some cases, the investigation is supplemented with TPO antibodies, which may indicate autoimmune thyroid disease.
In simplified terms, you can think of it this way:
High TSH + low free T4 indicates manifest hypothyroidism
High TSH + normal free T4 may indicate subclinical hypothyroidism
Normal tests mean that you need to consider other causes of the symptoms.
Sometimes tests need to be repeated. This is because slightly different values do not always mean chronic disease. Temporary variations may occur, and interpretation is affected by symptoms, medications, pregnancy and other medical conditions.
Many people wonder if it is enough to just test TSH. It is a very important screening test, but if it is abnormal, it usually needs to be supplemented with free T4 in order to assess the degree of hypofunction. If Hashimoto's disease is suspected, TPO antibodies are often valuable.
A common question is: “Can I have hypothyroidism despite normal test results?” If TSH and free T4 are normal, it usually speaks against common primary hypothyroidism, and then other causes of the symptoms should be looked for. In more unusual cases, there are other types of thyroid disorders, but they are significantly less common and need to be assessed by a doctor.
What causes hypothyroidism and who is at greater risk?
The most common cause is Hashimoto's thyroiditis, an autoimmune inflammation where the immune system attacks the thyroid gland. But there are several other causes.
Examples of causes and risk factors:
autoimmune thyroid disease
heredity for thyroid disease
previous thyroid surgery
previous radioiodine treatment
certain medications
other autoimmune disease
pregnancy and the period after childbirth
older age.
Women are affected more often than men. The risk is also higher if you or close relatives have autoimmune diseases. If you are already being treated for thyroid disease and become pregnant, your levels need to be monitored extra carefully, as the need for levothyroxine often increases during pregnancy. Untreated hypothyroidism during pregnancy can pose risks to both the pregnant woman and the fetus.
This means that certain groups should be especially alert to symptoms:
people with close relatives who have hypothyroidism or Hashimoto's disease
people with type 1 diabetes or another autoimmune disease
women with menstrual disorders or fertility problems
pregnant women or people planning a pregnancy
people who have previously been treated for another thyroid disease.
Treatment, follow-up and when you should get tested
The standard treatment for manifest hypothyroidism is levothyroxine, i.e. synthetic T4, which replaces the hormone that the thyroid gland no longer produces enough of. The treatment is well documented and is the first choice in guidelines for adults with primary hypothyroidism. The dose is adjusted individually and followed up with blood tests, especially TSH.
It is important to know that the improvement is not always noticeable immediately. Some symptoms are relieved within weeks, while others may take longer to recover from. Once the right dose is found, most people usually feel good, but follow-up is needed because both too low and too high a dose can cause problems.
You should consider testing your thyroid if you:
feel prolonged fatigue without a clear explanation
are unusually cold
have weight gain along with other typical symptoms
have constipation, dry skin or hair loss
experience brain fog, depression or difficulty concentrating
have menstrual disorders or fertility problems
are pregnant, planning a pregnancy or already being treated for hypothyroidism.
Seek care more quickly if you have a clear deterioration in your general condition, pronounced swelling, noticeable slowness, respiratory problems or if you are pregnant and suspect that your thyroid values are not in balance. Severe and long-term untreated hypothyroidism can in rare cases lead to serious complications.
For those who want to understand their health better, a blood test can be a first step towards clarity. If you also want to get a broader picture, it may also be relevant to supplement with other tests, such as iron status, B12, folate or Vitamin D depending on symptoms and context. This is a medically reasonable recommendation because several conditions can cause similar fatigue symptoms and sometimes occur in parallel.
