Mounjaro and an Underactive Thyroid: What People on Treatment Want to Know

An underactive thyroid means the thyroid gland produces insufficient thyroid hormone, which can slow metabolism and make weight management more difficult. Mounjaro (tirzepatide) is a prescription-only, once-weekly injectable medicine that acts on GLP-1 and GIP receptors involved in appetite regulation. These are two distinct physiological systems, but they can overlap in their effects on weight and energy — which is why your prescriber needs the full picture before and during treatment.

A note on thyroid-related cautions

The prescribing information for tirzepatide includes a caution relating to a specific type of thyroid tumour (medullary thyroid carcinoma) observed in animal studies. This does not mean Mounjaro causes thyroid problems in people, but it does mean your prescriber will consider your personal and family thyroid history when deciding whether it is suitable for you. If you have concerns about this, or any thyroid symptoms, raise them directly with your healthcare professional — they are best placed to assess your situation. You can also find factual prescribing information on the electronic Medicines Compendium (eMC).

Hypothyroidism and weight loss

Underactive thyroid weight loss can be slower and less predictable when thyroid function is not optimally managed, because low thyroid hormone levels affect the rate at which the body uses energy. Many people find that once their thyroid condition is well controlled — typically with levothyroxine or equivalent — weight management becomes more responsive. If you are on thyroid medication alongside Mounjaro, your prescriber may wish to monitor your thyroid function during treatment. This is a conversation worth having at your next appointment.

For broader context on how Mounjaro interacts with thyroid health, see our pages on Mounjaro and thyroid health and thyroid considerations with Mounjaro. If you are also managing another hormonal condition, our page on Mounjaro and endometriosis may be relevant reading.

When two conditions interact — each affecting energy, weight, and how you feel day to day — keeping clear records becomes genuinely useful. GLP Trackr's side-effect and wellbeing tracking is particularly valuable here: you can log fatigue, mood, and energy levels daily, building a picture of how you feel over weeks rather than relying on memory when you see your prescriber.

The daily check-in feature lets you record weight, energy, food noise, sleep quality, and mood in around 30 seconds each morning. Over time, this data surfaces patterns you might not notice otherwise — for example, whether certain weeks bring more fatigue, or how your appetite and energy track together. That kind of personalised record can make your prescriber appointments more productive, since you arrive with real data rather than a rough impression.

GLP Trackr also includes a doctor report feature, which pulls your logged data into a clear summary you can bring to your next appointment — useful when you are managing both a thyroid condition and a GLP-1 medicine and want to give your healthcare team the clearest possible view of your journey.

For more on what Mounjaro is and how it works, visit the Mounjaro overview hub.

Important: if you experience new or worsening symptoms — including a lump in your neck, persistent fatigue, changes in mood, or anything that concerns you — contact your healthcare professional promptly. GLP Trackr helps you record what you are experiencing so you can discuss it with your prescriber; it is not a substitute for their guidance, and no content here should be read as advice about your individual treatment.

If logging how you feel each day and bringing clearer records to your appointments sounds useful, GLP Trackr is free to try for 7 days — available on the App Store and Google Play.