u/TheThyroidDoc

A lot of posts here are people comparing two TSH numbers and panicking. Some of that swing is real. A lot of it is the draw itself.

Time of day. TSH follows a daily rhythm — highest overnight and early morning, lowest in the afternoon. An 8 AM draw and a 3 PM draw on the same person are not the same test.

Biotin. High-dose biotin (hair/skin/nail supplements) interferes with many lab immunoassays and can push results in the wrong direction. Most labs suggest stopping it a couple of days before a draw.

When you took your dose. Taking levothyroxine right before the draw can raise measured Free T4 without telling you much about your steady state. Most people are told to take the dose after the draw on lab days.

Illness, stress, and dieting. Acute illness and aggressive calorie restriction can shift thyroid labs temporarily. It's usually worth repeating a surprising result once you're well.

Different lab, different assay. Reference ranges and assays vary between labs. Comparing a Quest result to a LabCorp result isn't apples to apples.

Practical takeaway: keep the conditions the same every time — same lab, same time of morning, fasting, dose taken after the draw, biotin paused. Then the trend line actually means something.

A lot of posts here are people comparing two TSH numbers and panicking. Some of that swing is real. A lot of it is the draw itself.

Time of day. TSH follows a daily rhythm — highest overnight and early morning, lowest in the afternoon. An 8 AM draw and a 3 PM draw on the same person are not the same test.

Biotin. High-dose biotin (hair/skin/nail supplements) interferes with many lab immunoassays and can push results in the wrong direction. Most labs suggest stopping it a couple of days before a draw.

When you took your dose. Taking levothyroxine right before the draw can raise measured Free T4 without telling you much about your steady state. Most people are told to take the dose after the draw on lab days.

Illness, stress, and dieting. Acute illness and aggressive calorie restriction can shift thyroid labs temporarily. It's usually worth repeating a surprising result once you're well.

Different lab, different assay. Reference ranges and assays vary between labs. Comparing a Quest result to a LabCorp result isn't apples to apples.

Practical takeaway: keep the conditions the same every time — same lab, same time of morning, fasting, dose taken after the draw, biotin paused. Then the trend line actually means something.

I see a lot of confusion here about what each lab value means, so here's a simple breakdown:

TSH is not a thyroid hormone — it's the pituitary's "volume knob." High TSH usually means the pituitary is shouting at a sluggish thyroid.

Free T4 is the storage form your thyroid makes. Free T3 is the active hormone your cells actually use — most T3 is converted from T4 in the liver and elsewhere.

Antibodies (TPO, Tg) tell you whether an autoimmune process (like Hashimoto's) is attacking the gland — you can have positive antibodies years before TSH ever moves.

This is why "TSH only" screening can miss real problems.

If you're symptomatic (fatigue, cold intolerance, weight changes, brain fog, hair thinning) and only ever had TSH checked, it's reasonable to ask your doctor for a full panel: TSH, Free T4, Free T3, TPO antibodies.

Conversion from T4 to T3 can also be affected by stress, low-calorie dieting, and certain medications — which is why some people feel unwell with a "normal" TSH.

Happy to answer general questions in the comments. (General education, not medical advice — always work with your own physician.)

As a physician with over 50+ years in practice focused on thyroid and hormone health. One thing I see constantly is patients who spent years being told their fatigue was "just stress" or "just aging" before anyone ran a full thyroid panel.

Not just TSH — the full picture: Free T3, Free T4, TPO antibodies, thyroglobulin antibodies.

A few patterns I see over and over:

- **"I sleep 8+ hours and wake up exhausted."** If your TSH is "normal" but your Free T3 is low, you can still feel terrible. The standard reference range is wide, and "normal" doesn't always mean optimal.

- **"I can't lose weight no matter what I do."** Subclinical hypothyroidism tanks your metabolism before it shows up on a basic TSH screen. Many people diet and exercise hard with zero results because the underlying issue was never caught.

- **"My doctor said my labs are fine."** This is the most common one. A single TSH test isn't enough to rule out thyroid dysfunction. If your symptoms are real, push for the full panel.

If you're dealing with persistent fatigue, unexplained weight gain, brain fog, or hair thinning — and basic bloodwork came back "normal" — it might be worth asking specifically about a comprehensive thyroid workup.

Happy to answer questions if anyone has them!