Thyroid and weight: what an underactive thyroid really explains

“Could it be my thyroid?” is one of the most common questions in any weight conversation, and one of the most reasonable. It is also, most of the time, not the answer — and the reason matters, because the alternative explanations are the ones you can act on.

What hypothyroidism actually does to weight

An underactive thyroid slows metabolic rate, and weight does rise. But the magnitude is smaller than the reputation. The American Thyroid Association’s summary is that around 5 to 10 pounds (roughly 2 to 5 kg) may be related to the thyroid depending on severity, and that most of the weight gained is retained salt and water rather than fat ^[2].

That is a meaningful amount. It is not two stone.

The other symptoms are usually the more reliable clue: fatigue, cold intolerance, constipation, dry skin, hair thinning, low mood, heavy or irregular periods, slowed thinking.

The part that surprises people

Treating it does not usually reverse the weight.

A cohort of 101 adults with newly diagnosed primary hypothyroidism — TSH of 10 or above, median 18.3 — were followed until their TSH normalised on levothyroxine. The median weight change was −0.1 kg. Only about half lost any weight at all, and among those who did, the average loss was under 4 kg ^[3].

The authors put it plainly: contrary to popular belief, no significant weight change occurs after starting levothyroxine.

So if you have been waiting for treatment to undo the gain, that wait is the problem. Correcting the hormone deficiency is important for many reasons. Weight loss is not reliably one of them.

How it is tested in the UK

NICE guidance is specific ^[1]:

• Measure TSH alone first, where pituitary disease is not suspected
• If TSH is raised, measure FT4 on the same sample
• If TSH is suppressed, measure FT4 and FT3

Reference ranges are set by the individual laboratory and differ between assays, so the figure your result is compared against is local rather than national.

For overt hypothyroidism, levothyroxine is started at about 1.6 micrograms per kilogram per day in adults under 65 without heart disease, and lower with gradual titration in older adults or those with cardiac history. TSH is rechecked every three months until stable, then annually ^[1].

Subclinical hypothyroidism

Raised TSH with a normal FT4. This is where most of the argument happens.

NICE supports considering levothyroxine when TSH is 10 or above on two occasions three months apart, and considering a six-month trial in adults under 65 whose TSH is above range but below 10 and who have symptoms — stopping if symptoms do not improve ^[1].

In older adults the evidence is unfavourable. The TRUST trial found no benefit on symptoms or quality of life from levothyroxine in adults aged 65 and over with subclinical hypothyroidism ^[4].

Weight loss is not an indication for treating subclinical hypothyroidism, and no UK guideline supports it.

The thing not to do

Do not take thyroid hormone to lose weight if your thyroid function is normal. It does not produce useful fat loss, it does cause loss of muscle and bone, and at higher doses it causes atrial fibrillation. Thyroid hormone sold online as a slimming aid is one of the more dangerous things in that market.

If the tests are normal

That is worth treating as information rather than a dead end. The usual explanations for stalled weight loss — a smaller deficit than it feels like, weekends undoing weekdays, liquid calories, short sleep, a medicine you take — are covered in why am I not losing weight.

When to speak to a clinician

Ask for thyroid function tests if you have:

• unexplained weight gain with fatigue, cold intolerance, constipation or dry skin
• a slow pulse, hoarse voice, or puffiness around the eyes
• heavy or irregular periods with fatigue
• a family history of thyroid disease, or another autoimmune condition
• a neck swelling

Also speak to a clinician if you are already on levothyroxine and have not had a TSH check in the last year, or if your dose has changed and symptoms have not.

This article is for general information and isn’t a substitute for advice from your own clinician. If you feel unwell, contact your GP or NHS 111. In an emergency, call 999.