Thyroid and Weight Gain
Thyroid and Weight Gain: What’s the Real Connection?
The thyroid is a small, butterfly-shaped gland in the lower neck, but it has a big impact on how the body works. One of its most important roles is regulating metabolism—the rate at which the body burns energy. Because of this, thyroid disorders are often linked with changes in weight, particularly weight gain. However, the relationship is more complex than many people realise.
How the Thyroid Affects Metabolism
The thyroid produces two main hormones:
– T4 (thyroxine)
– T3 (triiodothyronine) — the active form
These hormones influence:
– how quickly calories are used
– heat production
– heart rate
– digestion
– fat and carbohydrate metabolism
When the thyroid slows down, the entire metabolic system slows with it.
Hypothyroidism and Weight Gain
Hypothyroidism, or an underactive thyroid, is the thyroid disorder most commonly associated with weight gain. It may be caused by:
– Hashimoto’s thyroiditis (autoimmune disease)
– previous thyroid surgery or radioactive iodine treatment
– medications (e.g., lithium, amiodarone)
– iodine deficiency (rare in developed countries)
Why does weight increase?
Weight gain in hypothyroidism is usually due to:
– slower metabolism
– salt and water retention (not just fat accumulation)
– fatigue and reduced activity levels
– changes in appetite
Most people gain 2–5 kg on average—less than commonly assumed.
Does Treatment Reverse the Weight Gain?
Standard treatment is levothyroxine (T4) replacement.
When thyroid hormone levels return to normal:
– some weight may be lost, mostly from water
– weight does not usually return fully to pre-illness levels
– persistent weight gain often has other contributing factors
If symptoms continue despite normal thyroid tests, it is important to:
– review dosing and timing of medication
– check for interfering medicines (e.g., iron, calcium, PPIs)
– consider other causes of weight change
Other Thyroid Conditions and Weight
Hyperthyroidism (overactive thyroid)
Often causes weight loss, but treatment (e.g., antithyroid medication or radioactive iodine) may lead to weight regain, sometimes above original weight.
Thyroid cancer and surgery
After total thyroidectomy:
– weight changes are usually minimal when TSH is well controlled
– overtreatment (suppressive doses) may cause weight loss
– undertreatment can contribute to weight gain
Subclinical hypothyroidism
Mildly elevated TSH with normal T4:
– may cause subtle symptoms
– treatment does not reliably cause weight loss
Is the Thyroid Always to Blame?
Not necessarily. Common overlapping causes include:
– ageing and hormonal changes
– stress and sleep disturbance
– menopause
– medications (e.g., steroids, antidepressants)
– reduced physical activity
– insulin resistance
A normal thyroid blood test generally means the thyroid is not the primary driver of weight gain.
When to Test Thyroid Function
Speak to a doctor if there is:
– unexplained weight gain with fatigue or cold intolerance
– dry skin, hair loss, or constipation
– menstrual changes
– family history of thyroid disease
– history of neck surgery or radioactive iodine
Testing includes:
– TSH
– Free T4
– thyroid antibodies if autoimmune disease is suspected
Supporting Healthy Weight With Thyroid Disease
Key strategies include:
– take levothyroxine on an empty stomach, away from calcium/iron
– maintain regular physical activity
– prioritise sleep and stress management
– balanced, nutrient-rich diet (not extreme calorie restriction)
– avoid unproven “thyroid booster” supplements
A tailored plan with a GP, dietitian, or endocrinologist is ideal.
Key Takeaway
Thyroid disorders can contribute to weight gain, but usually only modestly. Correct treatment improves metabolism and overall wellbeing, but weight changes are often influenced by multiple factors—not the thyroid alone. Early diagnosis and appropriate management remain the most effective approach.