Introduction
Medicines like Wegovy (semaglutide) and Mounjaro (tirzepatide) are increasingly used to support weight loss. If you already live with a thyroid condition such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid), you may wonder how these treatments could affect your thyroid.
While research has not shown that GLP-1 medicines directly change thyroid hormone levels, weight loss and metabolic changes can alter how much thyroid medication you need. For this reason, monitoring your thyroid function is an important part of your treatment journey.
How GLP-1 Medicines Work
Wegovy and Mounjaro belong to a group of medicines called GLP-1 receptor agonists (Mounjaro also acts on another hormone called GIP). They work by:
- Helping you feel full sooner and for longer.
- Slowing digestion so you eat less.
- Supporting your body to use insulin more effectively.
The result is weight loss, which brings many health benefits. However, because your metabolism changes with weight loss, your thyroid hormone needs may also change if you already take thyroid medication (1,2).
Do These Medicines Affect Thyroid Levels?
Thyroid hormone levels (TSH, T4, T3): Large clinical studies have found that GLP-1 medicines do not directly disrupt thyroid hormone production (3,4).
Patients with hypothyroidism: If you take levothyroxine, weight loss may lower your required dose, because thyroid replacement doses are partly based on body weight (5).
Patients with hyperthyroidism: There is no evidence that GLP-1 medicines worsen thyroid overactivity, but weight loss may influence how your treatment works, so regular monitoring is still advised (6).
What About Thyroid Cancer?
Animal studies suggested GLP-1 medicines might increase the risk of certain thyroid cancers. However, in human trials, there has been no proven link between GLP-1 treatment and thyroid cancer (3,7).
Even so, patients with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia type 2 (MEN2) are advised not to use these medicines (7,8).
What Patients Should Consider:
1. Before starting treatment
If you have a personal or family history of medullary cancer or MEN2, you should disclose this information, your GP or private weight loss provider will advise you if GLP-1 medicines are not suitable.
If you have a history of hypothyroidism or hyperthyroidism or any other thyroid disease, inform your GP or Endocrinologist about your intention of starting wegovy or mounjaro. This should allow them to arrange for a baseline thyroid blood test (TSH , TS3 , TS4 ) if they deem this necessary before you start weight loss treatment.
Disclose a full list of medications to your private weight loss prescriber.
2. During Treatment
Check thyroid bloods regularly (usually every 3-6 months, or sooner if symptoms appear). monitoring your thyroid function helps ensure your treatment stays safe and effective.
Watch for symptoms which may indicate your thyroid is out of balance:
- Low Thyroid (hypothroid) : Tiredness, feeling cold, low mood, constipation, dry skin and hair.
- Overactive thyroid (hyperthyroidism) : Palpitations , sweating , tremors , anxiety
Keep a symptom diary to share with your doctor.
Never stop or change your thyroid medication without medical advice.
3. Ongoing / After weight loss
Once your weight has stabilised, your thyroid medicine dose may also settle, but you should continue with regular monitoring.
Never change or stop thyroid medication without medical advice.
Maintain healthy habits; diet, exercise and sleep, which also supports thyroid balance.
4. When to seek Urgent Medical Attention
Call your doctor or seek urgent medical help if you notice a fast or irregular heartbeat, severe chest pain, sudden confusion or extreme tiredness, a rapidly growing lump in your neck, or persistent hoarseness.
Summary
GLP-1 medicines like Wegovy and Mounjaro do not usually affect thyroid hormone levels directly. However, because weight loss can change your body’s hormone needs, regular monitoring of TSH, T4, and sometimes T3 is important if you have hypothyroidism or hyperthyroidism.
For most people, these medicines are safe from a thyroid perspective. The main exception is for people with a history of medullary thyroid cancer or MEN2, who should avoid them.
With careful monitoring and support from your healthcare team, you can safely manage your thyroid condition while benefitting from your weight loss journey.
References
- Nauck MA, Meier JJ. Incretin hormones: Their role in health and disease. Diabetes Obes Metab. 2018;20 Suppl 1:5–21.
- Wilding JPH, Batterham RL, Calanna S, Davies M, Van Gaal LF, Lingvay I, et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med. 2021;384(11):989–1002.
- Chavda J, Lingvay I, Brown K, Perdomo G, Frias JP, et al. Tirzepatide and thyroid outcomes: Analyses from clinical trials. Diabetes Obes Metab. 2023;25(8):2059–68.
- Marso SP, Bain SC, Consoli A, Eliaschewitz FG, Jódar E, Leiter LA, et al. Semaglutide and cardiovascular outcomes in patients with type 2 diabetes. N Engl J Med. 2016;375(19):1834–44.
- Santini F, Pinchera A, Marsili A, Ceccarini G, Castagna MG, Lazar V, et al. Lean body mass is a major determinant of levothyroxine dosage in the treatment of thyroid diseases. J Clin Endocrinol Metab. 2005;90(1):124–7.
- Jonklaas J, Bianco AC, Bauer AJ, Burman KD, Cappola AR, Celi FS, et al. Guidelines for the treatment of hypothyroidism. Thyroid. 2014;24(12):1670–751.
- U.S. Food and Drug Administration (FDA). Wegovy prescribing information. 2023. Available from: https://www.accessdata.fda.gov/drugsatfda_docs/label/2023/215256s000lbl.pdf
- European Medicines Agency (EMA). Mounjaro: summary of product characteristics. 2022. Available from: https://www.ema.europa.eu/en/documents/product-information/mounjaro-epar-product-information_en.pdf