Hypothyroidism: Causes, Tests and Getting the Levothyroxine Dose Right
The thyroid sets the pace of the body. When it produces too little hormone, everything slows: heart rate, digestion, thinking, temperature, even the rate at which cholesterol is cleared from the blood. The symptoms are vague enough that people blame work, age or stress for years.
The good news is that hypothyroidism is one of the most treatable conditions in medicine: a single tablet a day, correctly dosed, restores normal function for most people. The catch is that how the tablet is taken matters almost as much as the dose, and a large share of people never get that part explained.
This guide covers the causes, what the blood tests actually mean, how levothyroxine is dosed and timed, pregnancy, and what to do when symptoms persist despite a normal test.
🧬 Why the thyroid slows down
| Cause | What happens | Who it affects |
|---|---|---|
| Hashimoto thyroiditis | The immune system gradually destroys thyroid tissue | The most common cause where iodine is sufficient; far more common in women |
| After thyroid surgery | Part or all of the gland removed | Thyroid cancer, large goitre |
| After radioiodine | Thyroid tissue destroyed deliberately | Treatment of an overactive thyroid |
| Medicines | Lithium, amiodarone, interferon, some cancer immunotherapies | Anyone on long-term treatment with them |
| Iodine | Too little, or sometimes too much | Mostly regions without iodised salt |
| Pituitary problem | The brain fails to signal the thyroid | Rare; needs different interpretation of tests |
| Congenital | Present from birth | Detected by newborn screening |
The distinction that changes everything is primary versus secondary. In primary hypothyroidism the gland itself is failing and TSH rises as the brain shouts louder. In secondary hypothyroidism the signal from the pituitary is missing, so TSH is low or normal despite a low thyroid hormone level. Treating the second kind without checking adrenal function first can be dangerous.
👀 Symptoms
Hypothyroidism rarely announces itself. It accumulates.
- 🔋 Tiredness that sleep does not fix
- 🥶 Feeling cold when others are comfortable
- ⚖️ Weight gain of a few kilograms, mostly fluid
- 🧴 Dry skin, brittle hair, hair thinning, including the outer eyebrows
- 🚽 Constipation
- 🩸 Heavy or irregular periods, and difficulty conceiving
- 🧠 Slowed thinking, low mood, poor memory
- 💤 A slow pulse, hoarse voice, puffy face
- 🫀 Raised cholesterol on a routine blood test, sometimes the first clue
🧪 Reading the blood tests
| Pattern | TSH | Free T4 | Meaning |
|---|---|---|---|
| Primary hypothyroidism | High | Low | The gland is underactive; treatment indicated |
| Subclinical hypothyroidism | Mildly high | Normal | Borderline; treatment depends on level, symptoms and antibodies |
| Secondary hypothyroidism | Low or normal | Low | Pituitary problem; needs specialist assessment |
| Non-thyroidal illness | Variable | Low-ish | Seen during serious illness; usually repeat when recovered |
Thyroid antibodies (anti-TPO) confirm an autoimmune cause and predict who will progress from borderline to definite hypothyroidism. They do not need repeating once known.
💡 Two practical points about testing. Take the blood sample before the morning tablet, not after, because levels peak a few hours later and can look falsely high. And high-dose biotin supplements, often sold for hair and nails, interfere with thyroid assays and can mimic either an overactive or underactive result. Stop biotin for two days before testing.
💊 Levothyroxine: the treatment
Levothyroxine is synthetic T4, identical to the hormone the thyroid makes. The body converts it to the active form, T3, as needed. It is one of the most prescribed medicines in the world, and it works when the dose and the routine are right.
| Situation | Usual approach |
|---|---|
| Healthy adult, clear hypothyroidism | Full replacement from the start, roughly 1.6 micrograms per kilogram of body weight daily |
| Over 65, or heart disease | Start low, typically 25 micrograms, and increase every few weeks to avoid straining the heart |
| Subclinical | A lower starting dose, often 25 to 50 micrograms |
| Retesting | TSH 6 to 8 weeks after starting or changing the dose, then yearly once stable |
| Target | TSH within the normal range, usually in the lower half; targets differ in pregnancy and in the very elderly |
⏰ How to take it, which is where most problems start
- On an empty stomach
- Usually 30 to 60 minutes before breakfast, with water. An alternative that works equally well is at bedtime, at least three hours after the last food.
- Coffee counts
- Coffee taken within an hour of the tablet noticeably reduces absorption. So does soya milk and a high-fibre breakfast.
- Four hours away from these
- Calcium and iron supplements, multivitamins containing them, antacids, sucralfate and some cholesterol binders. They bind levothyroxine in the gut.
- Acid matters
- Long-term omeprazole and similar acid reducers lower absorption, so the dose may need reviewing when they are started or stopped.
- Same brand where possible
- Small differences between products can shift TSH. If the brand changes, retest in six to eight weeks.
- Missed dose
- Take it when you remember the same day, or take two the next day; the hormone lasts about a week in the body, so an occasional miss is forgiving.
Levothyroxine is available in our thyroid category as Eltroxin (Levothyroxine) 50 mcg, Thyronorm (Levothyroxine) 88 mcg and Synthroid (Levothyroxine) 200 mcg. Dose changes should follow a blood test, not how you feel on a given day.
🔴 Too much is not better. Over-replacement causes palpitations, tremor, anxiety, insomnia, heat intolerance and weight loss, and over years it increases the risk of atrial fibrillation and bone thinning. If your TSH is suppressed below normal, the dose is too high even if you feel energetic on it.
🤰 Pregnancy
Thyroid hormone requirements rise early in pregnancy, often by 25 to 30 percent, and untreated hypothyroidism is linked to miscarriage, pre-eclampsia and effects on the baby's development.
- Tell your doctor as soon as you have a positive test — the dose is usually increased immediately
- Test TSH every four weeks in the first half of pregnancy
- Targets are lower than outside pregnancy
- Iron and calcium supplements, which most pregnant women take, must be kept four hours away from the tablet
- After delivery, the dose usually returns to the pre-pregnancy level, with a check six weeks later
⚖️ Subclinical hypothyroidism
A mildly raised TSH with a normal T4 is common, particularly with age, and it does not always need treatment. Treatment is usually offered when TSH is above about 10, when antibodies are positive and symptoms fit, during pregnancy or when trying to conceive, and sometimes as a three to six month trial in younger people with clear symptoms. In people over 80 with a modestly raised TSH, treatment often does more harm than good.
🤔 Still tired although TSH is normal
This is one of the most frequent frustrations in thyroid care, and it deserves a straight answer. A normal TSH means replacement is adequate. Persisting symptoms then usually have another explanation, and the list is worth working through:
- Iron deficiency, with or without anaemia, which is very common in women
- Vitamin B12 or vitamin D deficiency, both more common in autoimmune thyroid disease
- Coeliac disease, which travels with Hashimoto disease
- Sleep apnoea, depression or anxiety
- Another autoimmune condition, such as Addison disease, or diabetes
- Absorption problems from the timing issues described above
Combination treatment with T3 is requested often. Trials have not shown a consistent benefit over levothyroxine alone, and T3 carries a higher risk of heart rhythm problems and bone loss, so it remains a specialist decision rather than a routine option. Desiccated animal thyroid extract has inconsistent hormone content and is not recommended in guidelines.
⚠️ Untreated, and the emergency at the far end
Left untreated, hypothyroidism raises cholesterol and cardiovascular risk, causes infertility and heavy periods, and can lead to nerve entrapments such as carpal tunnel syndrome. At its extreme, severe untreated hypothyroidism can progress to myxoedema coma: profound cold, confusion, very slow pulse and reduced consciousness, usually triggered by infection, cold exposure or sedative medicines in an older person. It is rare and it is a medical emergency.
📞 When to see a doctor
- Tiredness with cold intolerance, weight gain and constipation — ask for a thyroid test
- Pregnancy or planning pregnancy while taking levothyroxine — promptly
- Palpitations, tremor or insomnia on treatment — likely too much
- Symptoms unchanged after three months at a stable, correct dose
- Starting or stopping omeprazole, iron, calcium or a new brand — retest in six to eight weeks
- Confusion, extreme cold or very slow pulse in an older person — emergency
❓ Frequently asked questions
What does a high TSH mean?
It means the brain is signalling harder because the thyroid is not producing enough hormone. A high TSH with a low free T4 indicates primary hypothyroidism. A mildly high TSH with a normal T4 is subclinical and may or may not need treatment.
How should levothyroxine be taken?
On an empty stomach with water, 30 to 60 minutes before breakfast, or at bedtime at least three hours after food. Keep calcium, iron and antacids four hours away, and avoid coffee within an hour of the tablet.
Does coffee affect levothyroxine?
Yes. Coffee taken within about an hour of the tablet measurably reduces absorption, as do soya milk and high-fibre breakfasts. Waiting an hour solves it.
How long until I feel better?
Some improvement often comes within two weeks, but the full effect takes six to eight weeks, which is also when TSH is retested. Dose adjustments follow the blood test rather than day-to-day symptoms.
Is levothyroxine for life?
Usually yes in Hashimoto disease and after surgery or radioiodine. Some causes, such as a temporary thyroiditis or a medicine effect, resolve, and treatment can then be stopped under supervision with retesting.
Why am I still tired if my TSH is normal?
A normal TSH means replacement is adequate, so other causes should be checked: iron, B12 or vitamin D deficiency, coeliac disease, sleep apnoea, depression, or absorption problems from how the tablet is taken.
Should I take T3 as well?
Trials have not shown a consistent advantage over levothyroxine alone, and T3 carries higher risks of heart rhythm problems and bone loss. It remains a specialist decision, not a routine option, and desiccated thyroid extract is not recommended because its hormone content varies.
Will treatment make me lose weight?
Usually a few kilograms, mostly fluid rather than fat. Levothyroxine is not a weight loss treatment, and taking more than you need is harmful.
What happens to the dose in pregnancy?
Requirements rise early, often by 25 to 30 percent, so the dose is usually increased as soon as pregnancy is confirmed, with TSH checked every four weeks in the first half of pregnancy.
Can supplements affect my thyroid test?
Yes. High-dose biotin, sold for hair and nails, interferes with thyroid assays and can produce misleading results. Stop it for two days before testing.
📑 Sources and editorial
- Clinical guidelines on the diagnosis and management of hypothyroidism
- Guidelines on thyroid disease in pregnancy and the postpartum period
- Trials and reviews of combination T4 and T3 therapy
- Studies of levothyroxine absorption, timing, food and drug interactions
- Guidance on biotin interference in thyroid immunoassays
- Related products: Eltroxin (Levothyroxine), Synthroid (Levothyroxine), thyroid category
- RXshop Editorial Team — reviewed by Daniel Kim, MD, Endocrinology
Medical Disclaimer: The information in this article is for educational and informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek guidance from a qualified healthcare provider with any questions you may have regarding a medical condition, and before starting, stopping or changing any medication.