Underactive thyroid and weight: what's really going on
The thyroid often gets the blame for weight problems. How big its influence really is, and what to watch for if you take thyroid hormones.
- thyroid
- hormones
- metabolism
In short
An untreated underactive thyroid slows metabolism and can lead to weight gain, often partly from fluid retention. Once well treated, it usually plays only a small role in weight.
Thyroid hormones are not a weight-loss aid and should only be taken as prescribed.
What happens with an underactive thyroid
Thyroid hormones control how fast the body uses energy. Without enough of them you often feel tired, feel the cold, get constipated and gain a little weight. A common cause is Hashimoto's thyroiditis.
With the right hormone dose, metabolism usually normalises. If weight is still an issue, the reasons often lie elsewhere: diet, activity, sleep, other medicines.
What to watch for
- Have hormone levels checked regularly, especially if your weight changes
- Take tablets as prescribed, usually on an empty stomach before breakfast
- Always tell your doctor and pharmacy about new medicines, as some affect absorption
- Mention thyroid hormones if you start GLP-1 treatment
Thyroid and GLP-1 treatment
A well-treated underactive thyroid usually doesn't rule out GLP-1 treatment. Because gastric emptying changes, absorption of oral medicines can be affected. For levothyroxine, the product information of some medicines says thyroid levels should then be monitored.
Certain thyroid conditions in your own or family history, such as medullary thyroid cancer, matter for the decision. Be sure to include them in your questionnaire.
Note
This Journal article offers general information. It is not a diagnosis, a prescription or a promise of any result. Whether treatment is suitable for you is decided by a doctor after reviewing your questionnaire. For acute or severe symptoms, contact your GP, the out-of-hours medical service (116117) or, in an emergency, 112.
Sources and medical review
Based on European Medicines Agency product information for GLP-1 medicines and hypothyroidism guidelines. Medical review pending.
- European Medicines Agency product information for semaglutide and tirzepatide (interactions, warnings)
- DEGAM guideline: Raised TSH in general practice