Underactive Thyroid
Hypothyroidism: the thyroid producing too little hormone, slowing many body processes.
Last revised
Key takeaways
- Main test: TSH, with free T4 if abnormal
- Pattern: high TSH with low T4
- Most useful single change: Take levothyroxine on an empty stomach at the same time every day.
- Commonest mistake: Taking it with breakfast, coffee or supplements.
- See a doctor if any of the signs under "When to get help" below apply to you.
Why it matters
Symptoms are vague and easily put down to aging, stress or low mood, and treatment is usually a single inexpensive daily pill.
The numbers, in one place
| Measure | Figure |
|---|---|
| Main test | TSH, with free T4 if abnormal |
| Pattern | high TSH with low T4 |
| Treatment | levothyroxine, usually taken daily for life |
| How to take it | on an empty stomach, 30-60 minutes before breakfast |
| Retest after a dose change | usually 6-8 weeks |
What helps
- Take levothyroxine on an empty stomach at the same time every day
- Separate it by about four hours from iron, calcium and antacids
- Have TSH rechecked 6-8 weeks after any dose change
- Tell your doctor as soon as you become pregnant; the dose usually needs increasing promptly
What to avoid
- Taking it with breakfast, coffee or supplements
- Adjusting the dose yourself
- Buying thyroid products online, including unregulated thyroid extracts
In depth
Your thyroid is a butterfly-shaped gland at the front of your neck. It makes mainly thyroxine (T4) and a smaller amount of triiodothyronine (T3), and your tissues convert T4 into the more active T3 as they need it. Together these hormones set the pace of your metabolism: how fast your heart beats, how warm you feel, how quickly your gut moves and how efficiently your cells use energy. When there is too little hormone, almost every system slows down a little, which is why the symptoms are so varied and so easy to miss.
The pituitary gland at the base of your brain watches thyroid hormone levels and releases thyroid-stimulating hormone (TSH) to push the thyroid harder when levels fall. In hypothyroidism the thyroid cannot keep up, so TSH climbs. In the US the most common cause is Hashimoto's thyroiditis, an autoimmune condition in which the immune system gradually damages the gland over months or years. Because the decline is slow, many people adapt to feeling tired and cold without realizing anything has changed.
Levothyroxine is a synthetic version of the T4 your gland would normally make. It replaces the missing hormone rather than forcing the thyroid to work, which is why it is usually needed long term and why it is so well tolerated at the right dose. The goal is to bring TSH back into range and relieve symptoms. Finding that dose often takes a few adjustments, and while some people feel better within weeks, others notice changes in skin, hair and energy only over several months.
Signs and symptoms
- Tiredness and sluggishness that sleep does not fix
- Feeling cold when people around you are comfortable
- Dry skin, coarse hair or thinning hair
- Constipation that is new or worse than usual
- Modest weight gain, often partly from fluid retention
- Slowed thinking, poor memory or low mood
- Heavier or more irregular menstrual periods
Causes and risk factors
- Hashimoto's thyroiditis, the most common cause in the US
- Previous radioactive iodine treatment or thyroid surgery
- Medicines that affect the thyroid, such as lithium and amiodarone
- Inflammation of the thyroid after pregnancy, which is sometimes temporary
- Being female, being older, or having a family history of thyroid disease
- Other autoimmune conditions such as type 1 diabetes or celiac disease
How it is checked
Testing usually starts with a TSH blood test, prompted by symptoms, pregnancy planning, a family history or medicines known to affect the thyroid. If TSH is high, free T4 is measured. A high TSH with a low free T4 confirms an underactive thyroid; a high TSH with a normal free T4 is called subclinical hypothyroidism, a milder pattern that does not always need treatment.
Your doctor may also check thyroid peroxidase (TPO) antibodies, which point to Hashimoto's thyroiditis and suggest a higher chance that a borderline result will progress over time. Because TSH can rise temporarily while you recover from an illness, a single slightly raised result is often repeated before any decision is made. Imaging is not usually needed unless your doctor feels a lump or an enlarged gland.
Related topics
- Testosterone: What the Evidence Actually Says (Men's Health)
- Type 2 Diabetes: Sorting the Evidence From the Noise (Diabetes)
- What the Guidance Really Says About Fatigue in Long-Term Illness (Long-Term Conditions)
- Menstrual Cycle Tracking: What Published Guidance Supports (Sexual Health)
Where to go next
- Underactive Thyroid: A Practical Checklist
- Underactive Thyroid: Separating Myth From Guidance
- Thyroid and Hormones: The Complete Guide
- Overactive Thyroid: Sorting the Evidence From the Noise
- What the Guidance Really Says About Thyroid Function Tests
When to get help
Contact your doctor promptly if you become pregnant while taking levothyroxine, and report palpitations, tremor or weight loss, which can indicate too high a dose. Call 911 for extreme drowsiness, confusion, very low body temperature or collapse, which can indicate severe untreated hypothyroidism.
Where this comes from
- American Thyroid Association — Hypothyroidism (Underactive)
- National Institute of Diabetes and Digestive and Kidney Diseases — Hypothyroidism (Underactive Thyroid)
How this page was put together
Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarized or generalized beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to your doctor, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 911.