With an underactive thyroid (hypothyroidism), the thyroid gland produces too few hormones (fT3 and fT4). These control the entire metabolism: energy expenditure, heartbeat, body temperature, digestion, mood, skin and hair. When there's a shortage, everything tends to run on a low flame.³
Hypothyroidism is one of the most common thyroid disorders — women are affected significantly more often than men, and it tends to become more common with age.³
The good news
The treatment is usually one of the simplest in medicine — one tablet a day (levothyroxine) replaces the missing hormone. With well-adjusted levels, most people can lead a completely normal life.
2. Causes: Hashimoto's and others
Most common causeHashimoto's thyroiditis — an autoimmune disease
The immune system slowly attacks and destroys the thyroid tissue over months to years. Typical antibodies: anti-TPO and anti-Tg. In rare cases, a temporary overactivity can even occur at the start (Hashitoxicosis).³
It runs in families. Hashimoto's not uncommonly occurs together with other autoimmune diseases: type 1 diabetes, celiac disease, vitiligo.
Other causes
After surgery or radioiodine therapy: When thyroid tissue has been removed or destroyed — e.g. after treatment for an overactive thyroid or a cancer. Levothyroxine is usually needed for life.
Medications: Amiodarone, lithium, interferon, certain immunotherapies (checkpoint inhibitors) and tyrosine kinase inhibitors can trigger an underactive thyroid. Have your thyroid levels checked regularly.
Iodine deficiency: Rare today in regions with iodized table salt — but still one of the most common causes worldwide.
Congenital hypothyroidism: Rare. It is detected through newborn screening and treated immediately.
Central hypothyroidism: Very rare. The problem lies in the pituitary gland (too little TSH) — TSH is then paradoxically low or normal despite a reduced fT4.
3. Symptoms
Symptoms usually develop gradually over months to years and are easily misinterpreted as aging, stress or burnout. Many people live with an undiagnosed underactive thyroid for a long time.³
Fatigue and lack of drive — often the most striking symptom; a leaden exhaustion that usually doesn't improve with sleep
Difficulty concentrating, forgetfulness
Weight gain despite no change in diet — due to a slowed metabolism and fluid retention
Sensitivity to cold
Dry skin, brittle hair and nails, diffuse hair loss
Constipation
Low mood, lack of drive — can be misdiagnosed as depression; with depression, always have the thyroid checked too
Muscle weakness, muscle cramps, joint pain
Menstrual cycle problems in women — heavier, longer or irregular bleeding
Fluid retention — e.g. a puffy face, swollen eyelids
A slowed heartbeat (bradycardia), raised cholesterol levels
A hoarse voice
4. Subclinical vs. overt hypothyroidism
SubclinicalSubclinical — TSH elevated, fT4 still normal
TSH is elevated, but fT3 and fT4 are still within the normal range. Symptoms are often mild or completely absent. Relatively common, especially in older women. Whether and when treatment is worthwhile is decided individually — taking into account how high the TSH is, any symptoms and antibody status.³˒⁴
OvertTSH elevated + fT4 reduced — levothyroxine usually clearly indicated
TSH is elevated and fT4 is reduced. Symptoms are usually present, and levothyroxine is usually clearly indicated.
A single elevated TSH is not enough for a diagnosis
A repeat check after a few weeks is usually recommended — the TSH level can be temporarily elevated by, for example, acute illness, stress or certain medications.
5. Diagnosis
TSH: The most important screening value — usually elevated in hypothyroidism.
fT3 and fT4: In overt hypothyroidism, fT4 is reduced; in the subclinical form it is still within the normal range.
TPO antibodies: Positive in the majority of Hashimoto's patients. They can be detectable years before an overt underactive thyroid develops.³
Tg antibodies: Less often positive, but also indicative.
Ultrasound: In Hashimoto's it often shows hypoechoic, inhomogeneous tissue, sometimes a shrunken thyroid. Nodules are also assessed.
Additional tests: Cholesterol (LDL is often raised in hypothyroidism), blood count, vitamin D, vitamin B12, iron/ferritin — accompanying deficiencies are common in Hashimoto's. Screening for celiac disease may be worthwhile.
Treatment usually consists of replacing the missing thyroid hormone in tablet form — with levothyroxine (L-thyroxine). With a permanent underactive thyroid, this is usually needed for life. Taking it correctly is crucial.³
How to take it correctly
In the morning on an empty stomach, some time before breakfast
With a glass of water
Always at the same time of day
Alternative: evening dose (a few hours after dinner) — after checking with your doctor
Missed dose: usually take it as soon as you remember
Keep a gap from
Coffee — not until some time after taking it
Milk and dairy products
Calcium, iron, magnesium — leave several hours' gap
PPIs (e.g. omeprazole, pantoprazole) — leave several hours' gap
The starting dose and the target dose are set individually — depending on factors such as age, body weight and other conditions. In older patients or those with heart conditions, the dose is usually increased especially slowly. TSH checks are usually done a few weeks after each dose change; once stable, at longer intervals.³˒⁴
Levothyroxine — liquid form (drops)
For swallowing difficulties or certain concurrent medications, the liquid form can be an advantage. Learn more: Generics vs. brand-name.
T3 combination therapy
Some patients don't feel completely well on levothyroxine despite normal TSH levels. A combination of T4 (levothyroxine) and T3 (liothyronine) can be considered in individual cases — current guidelines do not make a general recommendation for it. This decision is usually made by experienced endocrinologists. Thyroid extracts (e.g. from pigs) are generally not recommended by specialists.
7. Hashimoto's and mental strain
For many people, Hashimoto's is more than just a lab value. Even with well-adjusted levels, some report a considerable mental strain:³
Low mood and lack of drive — can be misdiagnosed as a separate depression; with depression, always have the thyroid checked too
Persistent exhaustion (fatigue), sometimes even with a normal TSH
Anxiety and mood swings
Frustration when symptoms aren't taken seriously because lab values are normal
Selenium for Hashimoto's
Selenium can be considered in consultation with your endocrinologist — there are indications that it can lower TPO antibodies and improve subjective well-being (moderate evidence). Overdosing should be avoided. Natural sources: Brazil nuts in moderation, fish, eggs.
Feeling unwell despite normal levels? Bring it up!
It can be worthwhile to additionally check fT3, iron/ferritin, vitamin D and vitamin B12 — accompanying deficiencies are common in Hashimoto's and can make symptoms worse.
8. Living with an underactive thyroid
Levothyroxine timing: Generally take it at the same time of day and under conditions that are as consistent as possible. A fixed routine helps — e.g. an alarm or a brite reminder.
Check-ups: TSH is checked at regular intervals. After a dose change, you usually wait a few weeks before measuring again.
Diet: A special Hashimoto's diet is usually not necessary. A balanced diet rich in selenium is usually sensible. Excessive soy consumption can impair levothyroxine absorption. Seaweed and kelp should generally be avoided with Hashimoto's. A gluten-free diet is only recommended if celiac disease has been confirmed.³
Exercise: Recommended, and it can improve mood, energy and weight. With a well-managed thyroid there are usually no restrictions.
Travel: Take your levothyroxine with you; refrigeration is usually not needed (it's stable at room temperature). With time-zone changes, usually take it at your usual local time. Learn more: Medication when traveling.
Myxedema coma — a rare emergency in severe untreated hypothyroidism
Signs: loss of consciousness, hypothermia. If suspected, call 112 immediately. It occurs almost exclusively in previously undiagnosed hypothyroidism.³
How brite helps you with an underactive thyroid
Levothyroxine in the morning on an empty stomach, in good time before breakfast — brite reminds you right on schedule and helps you keep an eye on your TSH levels and how you feel.
Medication reminders — Levothyroxine in the morning on an empty stomach, in good time before breakfast: brite reminds you right on schedule. Set up a reminder
Interaction check — Levothyroxine + calcium, PPIs, iron? Check the right gap for free. Check now
Health tracking — Record your TSH levels, weight, energy and symptoms over time. Track your history
Digital medication plan — all your medications clearly laid out for your endocrinologist and GP. Go to medication plan
FAQ: Common questions about an underactive thyroid
With Hashimoto's or after thyroid surgery, usually yes. With temporary forms (e.g. after thyroid inflammation), treatment can sometimes be time-limited. Levothyroxine should not be stopped on your own.³
Levothyroxine is best absorbed on an empty stomach. Food, coffee and certain medications (e.g. calcium, iron, PPIs) can significantly reduce its absorption. An alternative is an evening dose (a few hours after dinner) — after checking with your doctor's office.
At the correct dose, levothyroxine generally does not cause weight gain — it only replaces the missing hormone. The weight gain is usually a symptom of the underactive thyroid itself (slowed metabolism, fluid retention). With well-adjusted levothyroxine, weight often returns to normal.
Based on current knowledge, no. But the symptoms can usually be fully treated with levothyroxine. Selenium can be considered as an add-on in consultation with your endocrinologist. With well-adjusted levels, most people can lead a completely normal life.³
Possible reasons: the TSH may be within the normal range, but not necessarily in the range that is optimal for you. fT3 may be low. Accompanying deficiencies (iron/ferritin, vitamin D, vitamin B12) are common in Hashimoto's and can make symptoms worse. In these cases, a visit to an endocrinologist may be worthwhile.
TSH is elevated, but fT4 is still within the normal range. Symptoms may be mild or absent. Whether treatment is worthwhile is decided individually — taking into account symptoms, antibody status and how high the TSH is.³˒⁴
Selenium may possibly lower TPO antibodies and improve subjective well-being (moderate evidence). The dose should be discussed with your endocrinologist; overdosing should be avoided. Natural sources: Brazil nuts (in moderation), fish and eggs.
Only if celiac disease has been confirmed — Hashimoto's and celiac disease often occur together (screening may be worthwhile). Without celiac disease, based on current knowledge there is not enough evidence that a gluten-free diet improves Hashimoto's.
A rare, life-threatening emergency in severe, long-term untreated hypothyroidism. Signs include loss of consciousness and hypothermia. If suspected, call the emergency number 112 immediately. It occurs almost exclusively in previously undiagnosed hypothyroidism.³
Medical disclaimer: This article is for general information only and is not a substitute for medical advice, diagnosis or treatment. Levothyroxine should not be stopped or have its dose changed on your own. If a myxedema coma is suspected (loss of consciousness, hypothermia), call the emergency number 112 immediately. The dosage is always determined individually by your treating doctor. Last updated: April 2026.
