Thyroid checks for women: when weight will not shift

SASian Anderson, Writer and editor, MyGLP. Sources checked 21 October 2026.

What the thyroid does

The thyroid sits at the front of the neck and sets the pace of the body. When it is underactive, called hypothyroidism, everything slows: metabolism, gut, heart rate, mood. The NHS page says it affects about 1 in 50 women and is far more common in women than men. It becomes more common with age and after pregnancy.

The symptoms

• Tiredness.
• Weight gain, or weight that will not come off despite eating less.
• Feeling the cold.
• Constipation.
• Dry skin, thinning hair, brittle nails.
• Low mood, slow thinking.
• Heavy or irregular periods.
• Aches, cramps, weakness.

Half of those are side effects listed for the weight loss medicines. The other half are what brought you to a weight loss medicine in the first place. A woman who is tired, cold, constipated and losing hair on Mounjaro could have an underactive thyroid, low iron, both, or neither. The only way to know is to test.

The test

A blood test for TSH, thyroid stimulating hormone, is the first step. High TSH means the brain is shouting at a thyroid that is not keeping up. The GP may add free T4, the hormone itself, and thyroid antibodies, which show whether the cause is the immune system, which it usually is in the UK. The test is cheap, routine and often bundled with the full blood count and ferritin from the iron guide. Stop biotin supplements for a few days before it, since biotin can throw the result.

If you already take levothyroxine

Two things change on a weight loss medicine. First, your dose may need adjusting as you lose weight, because levothyroxine dose is partly set by body weight. Ask for a TSH check about 8 weeks after each significant drop, and at least once a year. Second, the medicines slow the stomach, and levothyroxine is sensitive to anything that changes absorption. NICE guidance on thyroid disease sets out the monitoring. Keep taking the tablet the same way every day: first thing, on an empty stomach, with water, 30 to 60 minutes before food, coffee or other medicines, and keep calcium and iron tablets at least 4 hours away. Consistency matters more than the exact routine.

The thyroid warning on the leaflets

Every GLP-1 leaflet carries a warning about thyroid C-cell tumours. The warning comes from rodent studies, where very high doses caused tumours in a type of thyroid cell. The same effect has not been shown in people, and the UK and European regulators did not make it a reason to refuse the medicine. The leaflets do say to tell the prescriber if you or a close relative have had medullary thyroid cancer, which is rare, or a condition called MEN2. An ordinary underactive thyroid is not that, and does not stop you taking the medicine. Any new lump in the neck, hoarseness that lasts, or trouble swallowing should be checked by a GP whatever you take.

An overactive thyroid

The opposite problem, hyperthyroidism, causes weight loss without trying, a racing heart, sweating, anxiety, loose stools and light periods. If weight is falling faster than the medicine and your eating would explain, or your heart races at rest, say so. The same TSH test picks it up, with a low result rather than a high one.

Pregnancy and the thyroid

An underactive thyroid makes it harder to conceive and raises the risk of miscarriage if untreated. If you plan to stop a weight loss medicine to try for a baby, which our fertility guide explains, get the thyroid checked at the same time. Women on levothyroxine usually need a higher dose as soon as they are pregnant.

Sources

1. Underactive thyroid (hypothyroidism) (NHS)
2. Thyroid disease: assessment and management (NG145) (NICE)
3. Levothyroxine (NHS)
4. Tirzepatide: who can and cannot use it (NHS)
5. Wegovy patient information leaflet (Novo Nordisk, via the electronic medicines compendium)