Re Blood tests

[Ashupan]

I am under the care of nuclear team as having RAI treatment after papillary cancer n Thyroidectomy.

I read various posts here and am now mystified and concerned.

The nuclear team and thyroid oncologist made it 100% clear , that prior to ANY blood tests, if one is taking thyroxine, it is ESSENTIAL to take the thyroxine, morning of test.

This gives a true indication of all levels.

Why would folk write its best not to take thyroxine before blood tests?

[PurpleNails Administrator]

Most dr don’t advise either way. They think testing at any time makes no difference. When looking at TSH the replacement dose does not have an immediate effect & most drs go by the TSH measure.

It’s advised to delay dose, so the daily FT4 is shown at lowest point during day.

Taking replacement a few hours before test would show peak daily level.

Sounds like your dr is wanting to see FT4 at highest peak, it might also be to check absorption.

if dr is accounting for when dose is taken & testing TSH, FT4 & FT3 there shouldn’t be an issue.

[tattybogle]

WHEN TO TAKE LEVO BEFORE BLOOD TESTS.

In case you , (or your GP/Endo) don't understand why we recommend to delay taking the morning dose of Levo until after a blood test at 9 am :

~ This paper confirms it is 'best practice' for GP's to test thyroid blood tests (TSH and fT4) at a consistent time of day and BEFORE taking that days Levothyroxine dose : onlinelibrary.wiley.com/doi...

Volume 2013 | Article ID 831275 | doi.org/10.1155/2013/831275

General Error Analysis in the Relationship between Free Thyroxine and Thyrotropin and Its Clinical Relevance Simon L. Goede and Melvin Khee-Shing Leow

(* note ~ Thyrotropin= TSH , and L-T4 = Levothyroxine )

"2.1. Diurnal Variations in [TSH]

Biological variations are expected to occur in the [FT4] and [TSH]. A very important condition for TFT measurements is the time of day the blood is sampled as many hormone systems in the body exhibit natural circadian biorhythms dependent on time including the HPT axis. The diurnal rhythm of [TSH] plays an important role and exhibits a significant difference between morning and evening readings [8]. The reported variations in [TSH] levels are ranging from an average [TSH] of 1 mU/L at about 15.00 h in the afternoon to an average of [TSH] = 2 mU/L at about midnight. This reveals the importance of a repeatable defined measurement regime at a fixed time of day. Any measurement accuracy of [TSH] to the extent we have available loses significance if we ignore these effects.

The amount of variability is dependent on the individual in question, but the smallest interindividual variations in [TSH] are observed around 15.00 h in the afternoon [8].

In another study [9], it was evident that several persons being probed for FT4 after taking their daily dose of levothyroxine (L-T4) had different readings because of interindividual differences in pharmacokinetics and metabolism. Therefore it is important to probe a person already using L-T4 on a fixed time of the day before the intake of the daily L-T4 dose. For practical reasons, this can be done shortly upon awakening (i.e., prior to the ingestion of daily dose of L-T4) in the early morning between 07.00 h and 10.00 h. The same time interval for TFT assessment also applies to people being investigated for the first time."

See image below :

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ncbi.nlm.nih.gov/pmc/articl... Increased fT4 concentrations in patients using levothyroxine without complete suppression of TSH (Heleen I Jansen, Marijn M Bult, Peter H Bisschop, Anita Boelen, Annemieke C Heijboer,Jacquelien J Hillebrand 2023)

"Since 72–81% of the studied patients with discordant fT4 + TSH results used L-T4, literature was researched to explain this phenomenon with a specific focus on time of blood withdrawal and time of L-T4 intake. Multiple studies have investigated serum fT4 concentrations directly after L-T4 ingestion, all including hypothyroid patients treated with a stable dose of L-T4. These studies all reported an equivalent course of fT4 after morning L-T4 intake (3, 4, 5, 6, 7, 8, 9, 10). Figure 2 depicts a summary of this literature search; it shows that fT4 concentrations rise after 1 h and peak between 2 and 4 h after L-T4 intake (+15–25%) followed by a gradual decline and return to baseline within 24 h in accordance with the known time to maximal concentrations of L-T4 (T-max = 2–3 h) ..... Timing of blood withdrawal following L-T4 intake can lead to high fT4 concentrations without (complete) TSH suppression. Hypothyroid patients are mainly treated with L-T4 and the effect of treatment is monitored by measuring serum TSH, sometimes accompanied by fT4. Hypothyroid patients are advised to take a single daily dose of L-T4 orally in a fasting state. L-T4 administration in the morning or at bedtime is considered equally effective as long as L-T4 is taken on an empty stomach to ensure optimal uptake (14, 15). In contrast to fT4, no direct alterations of TSH and T3 have been reported directly after L-T4 ingestion (4, 5, 6, 7, 8). An fT4 course as Fig. 2 presents was found in patients taking L-T4 in the morning before breakfast, and one would also expect an increase in fT4 levels during the night when L-T4 is taken at bedtime (16). However, literature on extensive follow-up of fT4 and TSH levels following L-T4 intake in the morning compared to bedtime is lacking. Ain et al. (7) as well as Hoermann et al. (17) specifically emphasized that fT4 concentrations in L-T4 users were influenced by the time of day, meaning the time interval between L-T4 intake and blood sampling should be considered in the interpretation of fT4 values. In line with this advice, the European Thyroid Association guideline on treating central hypothyroidism advises blood withdrawal for monitoring treatment to be performed before L-T4 intake or at least 4 h after L-T4 intake (18), but other international guidelines do not yet ..... Physicians and laboratory specialists should be aware of the importance of timing of blood withdrawal and the timing of L-T4 intake to avoid questioning the assay’s performance or, worse, unnecessarily adapting L-T4 dose in patients. "

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