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Condition Comparison

Hypothyroidism vs Hyperthyroidism

Same symptoms, different conditions — here's exactly how to tell them apart.

Hypothyroidism

An underactive thyroid gland that produces too little thyroid hormone. The body's metabolism slows down, causing weight gain, fatigue, and cold intolerance. Very treatable.

VS

Hyperthyroidism

An overactive thyroid gland that produces too much thyroid hormone. The body's metabolism speeds up, causing weight loss, anxiety, and heat intolerance. Requires prompt treatment.

Side-by-Side Comparison

FeatureHypothyroidismHyperthyroidism
Thyroid activityUnderactive — too little hormone (T3, T4)Overactive — too much hormone (T3, T4)
TSH levelHIGH TSH (pituitary is working hard to stimulate a failing thyroid)LOW TSH (pituitary is suppressed by excess hormone)
Weight effectWeight gain despite normal dietWeight loss despite normal or increased appetite
EnergyPersistent fatigue, lethargy, brain fogRestlessness, anxiety, insomnia
TemperatureCold intolerance — always feeling coldHeat intolerance — always feeling hot, excess sweating
Heart rateSlow pulse (bradycardia)Fast pulse (tachycardia); palpitations; atrial fibrillation risk
BowelsConstipationFrequent bowel movements or diarrhoea
Menstrual effectHeavy, irregular periodsLight periods or absent periods (amenorrhoea)
Skin/hairDry skin, coarse hair, hair loss, brittle nailsWarm, moist skin; fine hair; hair thinning
Goitre (neck swelling)Possible (Hashimoto's, iodine deficiency)Common (Graves' disease, toxic goitre)
Most common causeHashimoto's thyroiditis (autoimmune); post-surgicalGraves' disease (autoimmune); toxic nodular goitre
TreatmentDaily levothyroxine (lifelong in most cases)Carbimazole/propylthiouracil; radioiodine; thyroidectomy

Symptoms They Share

⚠️ Both conditions can cause:

The Key Difference

💡 How to tell them apart

Think of the thyroid as controlling the body's speed dial. Hypothyroidism turns it down — everything slows: metabolism, heart rate, bowels, thinking, and mood. The signature is unexplained weight gain with fatigue despite not eating more, combined with feeling cold when others are comfortable. Hyperthyroidism turns it up — everything races: heart rate, metabolism, reflexes, and anxiety. The signature is unexpected weight loss despite a good appetite, with a rapid heartbeat and heat intolerance. The TSH blood test settles the question immediately: high TSH = underactive; low TSH = overactive.

Which Tests Diagnose Each?

🧪 TSH (Thyroid Stimulating Hormone)
The single most important thyroid test. TSH is produced by the pituitary gland to control the thyroid. High TSH = pituitary is pushing hard because the thyroid is lazy (hypothyroid). Low TSH = pituitary is suppressed because the thyroid is overproducing (hyperthyroid).
🧪 Free T4 and Free T3
Measure the actual thyroid hormones in circulation. Used alongside TSH to confirm diagnosis and severity. Low free T4 confirms hypothyroidism; high free T4/T3 confirms hyperthyroidism.
🧪 Thyroid Antibodies (Anti-TPO, Anti-TG, Anti-TSH receptor)
Identify autoimmune causes: Anti-TPO and Anti-TG antibodies are elevated in Hashimoto's (hypothyroid). TSH receptor antibodies (TRAb) confirm Graves' disease (hyperthyroid).
🧪 Thyroid Ultrasound
Maps the size, texture, and nodularity of the thyroid gland. Detects goitre, nodules, or cysts that explain the dysfunction. Available same-day at Mascot Healthcare.

Frequently Asked Questions

Can my thyroid problem go away on its own?
Hypothyroidism almost never resolves spontaneously — Hashimoto's disease progressively destroys thyroid tissue. Some cases of hyperthyroidism (particularly in young women after pregnancy — postpartum thyroiditis) do resolve after 6–12 months. Graves' disease can go into remission with medication but often recurs.
Is hypothyroidism the reason I can't lose weight?
Hypothyroidism contributes to weight gain, but rarely more than 2–5 kg of fluid and fat. If you've gained 20 kg, the thyroid is probably not the only explanation. However, treating hypothyroidism is essential regardless — it affects heart health, cholesterol levels, and fertility even when weight is not the presenting concern.
Can I get pregnant with a thyroid condition?
Yes, but both conditions affect fertility and pregnancy. Untreated hypothyroidism increases the risk of miscarriage and affects baby brain development. Untreated hyperthyroidism raises the risk of preterm birth and fetal growth restriction. Both are entirely manageable in pregnancy with monitoring and appropriate treatment — inform your obstetrician about any thyroid history.
How quickly does levothyroxine work for hypothyroidism?
Most patients notice improvement in energy and mood within 4–6 weeks of starting levothyroxine at the right dose. The TSH level takes 6–8 weeks to stabilise after a dose change. Thyroid function should be rechecked 6–8 weeks after starting or changing the dose, then annually once stable.

Related Pages

Thyroid Function Lab StoryThyroid Scan Story

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