Thread by @karthik2k2

Someone had asked whether sampling for thyroid function test should be done in fasting only in the previous tweet. It has a short answer and a long answer.

1. Short answer - Not necessary for all
2. Long answer - this 🧵

Thyroid hormone levels aren't static throughout the day. There's short term day to day variability and longer term seasonal variability. This rhythm also varies between individuals - some have strong rhythmicity, others don't. So the impact of timing varies between individuals.

The within day variability (circadian rhythm) is more pronounced with TSH. See the curve with TSH and the relatively flat line with T4

Now why would fasting or fed state matter at all ?

Postprandial rise in somatostatin can inhibit TSH release and can potentially lead to a reduction in TSH. But the post prandial sample is also given at a later time that fasting sample. So is TSH ⬇ due to food or time or both?

This study (done by my senior colleague) is the only one that tested whether food or time or both mattered. They also looked at 3 different assay methods. Day 1 8 AM and 10 AM samples of TSH (extended fasting) showed reduction of TSH. Day F vs 2 hr PP TSH showed even more ⬇

So both timing and food intake contribute to reduction in TSH. So it is a good idea for labs to mention fasting state and time of sampling.

Does that mean all people should give fasting sample at or before 8 AM ?

Just reduction in TSH is not a deal breaker - but if it results in change in diagnosis, it is. Reclassification (normal vs subclinical hypothyroid ) happens when the baseline TSH is near the cut off.

If the baseline TSH is not near the cut off for normalcy(4 mIU/L) , then reclassification is rare (eg. only 6% in a study Scobbo W V Med J. 2004 Jul-Aug;100(4):138-42). Whereas if it is borderline TSH, reclassification can be more (Mahadevan et al)

This reclassification becomes more important in 1st trimester of pregnancy, as the cost of missing a mild thyroid problem is higher and borne by the baby. For non pregnant adults, especially men, this isn't as critical

So timing and fasting matter more to pregnant women and women in reproductive age who are planning pregnancy. For others, the small reduction in TSH won't matter much.

Summary :

- Ideal way to test thyroid function- fasting, before 8AM.
- Matters for TSH only
- Candidate for 'ideal conditions' testing - pregnant woman, ♀ planning pregnancy
- Inter individual, intraday and longer term variability should also be kept in mind