Two witnesses: your symptoms and your TSH blood test - checked 6 to 8 weeks after any dose change (TSH needs that long to settle) and yearly when stable, per the NHS.
Underdose feels like your old hypothyroid self: fatigue, weight creep, cold intolerance, constipation, fog. Overdose mimics hyperthyroidism: racing or irregular heartbeat, shakiness, sweating, anxiety, diarrhea, poor sleep, headaches, weight loss. The fakes: taking it with food, coffee, or other medicine (all blunt absorption - empty stomach, 30 to 60 minutes before first meal or caffeine), calcium and iron within 4 hours, and biotin supplements, which distort the lab itself. Never self-adjust on a good or bad week - symptoms plus TSH, read together, drive changes. Pymander is a free AI doctor, 24/7 by text, and can help you read the pattern.
What to do
Common questions
How do you know if your levothyroxine dose is right?
Your dose is right when two witnesses agree: how you feel, and your TSH. The blood test is the objective witness - the NHS says you will have regular blood tests to check you are on the right dose, typically 6 to 8 weeks after any dose change (TSH takes that long to settle) and then once a year when stable. The felt witness is your daily life: an underdose leaves hypothyroid symptoms running - fatigue, weight creep, cold intolerance, constipation, brain fog, low mood; an overdose mimics hyperthyroidism - racing or irregular heartbeat, shakiness, sweating, anxiety, diarrhea, sleep problems, headaches, and unexplained weight loss; the NHS confirms being on the wrong dose causes side effects like these. The traps that fake a wrong dose: taking it with food, coffee, or other medicine (all blunt absorption - the rule is empty stomach, 30 to 60 minutes before your first meal or caffeine); inconsistent timing; calcium and iron supplements within 4 hours; and biotin supplements, which can distort the lab result itself - stop biotin a few days before the draw and tell the lab. The rule above all: never adjust the dose yourself based on a good or bad week - symptoms plus TSH, interpreted together, drive changes. Pymander is a free AI doctor, 24/7 by text, that can help you read the pattern.