TSH has a quite important circadian rhythm.
Most of the original research on it is not freely accessible, as open access wasn't really a thing when these studies were done, in the 80s, but this one is freely available: Physiological variations in thyroid hormones: physiological and pathophysiological considerations. There is a section on circadian and seasonal rhythm and a table with some values. Quoting from the paper:
There is a clear circadian variation in circulating TSH concentrations in animals and in humans. TSH concentrations are low during the daytime, increase in the evening, and peak shortly before sleep. Amounts decline slowly during sleep. Table 3 summarizes studies of mean, nadir, and peak serum
They also mention that the nocturnal TSH secretion is weakened in people with hyperthyroidism.
A chart is available in figure 2 of the paper Modulation of Endocrine Function by Sleep-Wake Homeostasis and Circadian Rhythmicity
Significance of latent hyperthyroidism is another study from the 80s on this. They found a significant circadian rhythm in normal (euthyroid) patients, but a suppressed rhythm in subjects with hyperthyroidism, the suppression getting stronger the worse the hyperthyroidism is:
The circadian rhythm in latent hyperthyroidism is distinctly suppressed and in overt hyperthyroidism totally. Whereas in latent hyperthyroidism pulsatile secretion is extant, in overt hyperthyroidism the TSH pulses are absent.
The same is basically observed in hypothyroidism, see, for example Diurnal thyrotropin secretion in short-term profound primary hypothyroidism: does it ever persist?
In conclusion, diurnal rhythmicity in serum TSH levels was abolished in a uniform cohort of patients with short-term severe primary hypothyroidism.
or Circadian changes in pulsatile TSH release in primary hypothyroidism.
The nocturnal TSH surge was absent in six out of nine patients with overt primary hypothyroidism.
The circadian rhythm is weaker, different, or even absent.
While I have always had doctors recommend having TSH done in the morning, without eating beforehand, this doesn't seem to be a general suggestion. However, it's at least desirable to always have them done at the same time of day, to make values comparable:
TSH has a circadian rhythm (24 hour cycle) and levels peak between midnight and 6am. T3 has a similar circadian rhythm. It is therefore a good idea to have your thyroid tests done at the same time of day each time as your levels may differ at different times of the day.
- primary hypothyroidism means that the hypothyroidism is caused by a thyroid disorder itself
- secondary hypothyroidism is hypothyroidism caused by, for example a problem in the part of the brain controlling the thyroid)
- an euthyroid patient has normal thyroid function
- latent thyroid problems are those that are detected in lab tests, but don't show symptoms yet
- subclinical is the reverse: showing symptoms, but blood tests have no significant results