Thyroid function monitoring on GLP-1 — TSH and free T4 trends

I work in an endocrinology clinic and I see this pattern a lot in patients on GLP-1 agonists AND caloric restriction. Let me break down what's likely happening: 1. The TSH rise from 1.8 to 3.6 is almost certainly related to caloric deficit, not the GLP-1 directly. When you eat significantly less, your body downregulates thyroid hormone production as a metabolic conservation mechanism. This is well-documented in weight loss literature regardless of method. 2. Your Free T3 dropping from 3.1 to 2.5 supports this. T3 is the active thyroid hormone and it's very sensitive to caloric intake. The body reduces T4-to-T3 conversion during energy deficit. 3. The black box warning is about medullary thyroid carcinoma (MTC), which is a completely different issue. MTC would show up as a thyroid nodule and/or elevated calcitonin, NOT as TSH changes. The rodent studies showed C-cell hyperplasia at doses 2-10x human equivalents. That said — your concern about the trend is valid. I'd recommend: - Get calcitonin checked for peace of mind (likely normal) - Ensure adequate iodine and selenium intake - Recheck in 3 months - If TSH exceeds 4.5 or you develop symptoms (fatigue, cold intolerance, hair loss, constipation), push for further workup

Thank you, that actually helps a lot. The distinction between TSH changes from caloric deficit vs. the MTC warning is something my doctor didn't explain at all. He just said "it's fine." I AM eating way less — probably 1100-1300 cal/day which I know is low. And yes I've noticed more hair shedding and feeling colder than usual. Is that the thyroid or just the weight loss?

> probably 1100-1300 cal/day That's almost certainly too low and is likely driving the thyroid suppression AND the symptoms you're describing. Hair shedding during rapid weight loss is extremely common — it's called telogen effluvium and it's related to caloric deficit, protein intake, and yes, thyroid function. My recommendation: try to get to at least 1400-1500 cal/day with minimum 80-100g protein. This alone often stabilizes thyroid function. Your body is currently in conservation mode.

Jumping in because I have Hashimoto's thyroiditis AND I'm on tirzepatide. I was terrified to start GLP-1 therapy given my existing thyroid condition. Here are my numbers for anyone in the same boat: | Test | Pre-Tirz | 3 Mo | 6 Mo | 9 Mo | |------|---------|------|------|------| | TSH | 3.2 | 3.8 | 3.5 | 3.1 | | Free T4 | 1.3 | 1.2 | 1.3 | 1.4 | | Free T3 | 2.9 | 2.6 | 2.8 | 3.0 | | TPO Antibodies | 248 | 210 | 185 | 142 | On levothyroxine 75 mcg throughout. The interesting thing is my TPO antibodies actually dropped significantly — from 248 to 142. My endo thinks the reduction in systemic inflammation from weight loss is calming the autoimmune attack on my thyroid. Several papers support this. So for Hashimoto's patients: in my n=1 experience, GLP-1 therapy actually IMPROVED my thyroid autoimmunity.