Falling ferritin is not always falling iron

The Journal groups these into three categories, and finds the classification more useful than any list. There are results altered by the physiology of energy deficit — the fall in triiodothyronine, the fall in insulin, the rise in ketones and free fatty acids, the shift in bile acid handling. There are results altered by the change in body composition, principally creatinine and the estimates built on it. And there are results altered by the immediate circumstances of the sample: haemoconcentration from reduced intake or vomiting, which raises creatinine, urea, albumin, haematocrit and calcium together, and which resolves with rehydration.

The eight things that break the HbA1c assay

Iron deficiency raises HbA1c independently of glycaemia, by a mechanism involving altered erythrocyte turnover and glycation kinetics; correcting the deficiency lowers it without any change in glucose. Any cause of accelerated erythrocyte turnover — haemolysis, recent transfusion, treatment of a deficiency anaemia, erythropoietin therapy — introduces young cells and lowers the value. Chronic kidney disease shortens erythrocyte survival and lowers it. Splenectomy raises it by extending survival.

Haemoglobin variants, including the common sickle and haemoglobin C traits, interfere with some assay methods, though not with all: ion-exchange chromatography and immunoassay behave differently and a laboratory should state its method when a variant is known. Pregnancy lowers it. Severe hypertriglyceridaemia and hyperbilirubinaemia interfere with some platforms.

Several of these are common in the population taking these drugs. Iron deficiency in particular is prevalent among people eating substantially less, and it pushes HbA1c in the direction that would make glycaemic control look worse than it is. A rising HbA1c during otherwise successful treatment is a reasonable prompt to check a full blood count and iron studies before concluding anything about glycaemia. That is an observation about assay behaviour and not clinical advice.

The creatinine artefact, stated precisely

Serum creatinine is the breakdown product of creatine phosphate in skeletal muscle, produced at a rate approximately proportional to muscle mass and cleared predominantly by glomerular filtration. Estimated glomerular filtration rate is calculated from serum creatinine with adjustments for age and sex, which function as population-average proxies for muscle mass.¹

When actual muscle mass falls, creatinine production falls, serum concentration falls, and the equation reports a higher estimated filtration rate. The magnitude is not trivial: a loss of four to five kilograms of lean tissue can shift estimated filtration rate upward by several millilitres per minute per 1.73 square metres with no change in the kidney whatever. The effect runs in the reassuring direction, which is why it is rarely questioned.

The check is cystatin C, a low-molecular-weight protein produced by all nucleated cells at a rate largely independent of muscle mass. Where creatinine-based and cystatin C-based estimates diverge substantially during rapid weight loss, the divergence is itself informative, and combined equations using both are available and better validated than either alone. Cystatin C has its own confounders — corticosteroids, thyroid dysfunction and adiposity all affect it — which is why the recommendation is to read the two together rather than to substitute one for the other.

The lipid panel: what moves, and because of what

During substantial weight loss on these agents, triglycerides fall markedly — reductions of the order of twenty per cent are reported in the obesity programmes — high-density lipoprotein cholesterol rises modestly, and low-density lipoprotein cholesterol falls only slightly.² That pattern is the signature of weight loss and improved insulin sensitivity rather than of a lipid-lowering drug effect, and it is worth saying so, because the class is sometimes described as though it were one.

Two measurement points matter. Triglycerides have large within-person biological variation, with a reference change value above thirty per cent, so an individual fall of twenty per cent between two panels may be noise even though the group mean fall of twenty per cent in a trial is a solid finding. And fasting is no longer required for routine lipid assessment; non-fasting samples differ trivially for total and LDL cholesterol and modestly for triglycerides, and international consensus has favoured non-fasting measurement for a decade.³

Lipoprotein(a) is worth a separate sentence because it is the exception. It is largely genetically determined, changes little with weight loss, and if it is going to be measured at all it needs measuring once rather than monitored. A person expecting it to improve alongside everything else will be disappointed by a result that was never going to move.

The acute-phase reactants move together

C-reactive protein falls substantially during successful weight loss on these agents, with reductions of the order of a third to a half reported in the obesity programmes, and it fell on treatment in the cardiovascular outcome trial in overweight and obesity without diabetes as well.⁴ Ferritin, fibrinogen and several other acute-phase proteins move in the same direction for the same reason: adipose tissue is an inflammatory organ and there is less of it.

The interpretive trap is ferritin. It is used clinically as a marker of iron stores and it is also an acute-phase reactant, which means it is raised by inflammation independently of iron. A person whose ferritin falls from 140 to 55 during a year of treatment may have depleted their iron stores on a reduced intake, or may have had a falsely reassuring ferritin all along that is now revealing a pre-existing deficiency, or may simply have less inflammation. Transferrin saturation and, where necessary, soluble transferrin receptor distinguish these; ferritin alone does not.

The same logic applies in reverse to any marker that is suppressed by inflammation. The Journal reports these movements as a group because reading them individually is how the mistakes happen: three or four analytes moving together during weight loss is a single physiological story, and treating it as three or four findings multiplies the investigation without adding to the information.

The panel drawn on a bad week

A person in the middle of a difficult dose escalation may be eating little, drinking less than usual and vomiting intermittently. A panel drawn in that state measures the state. Reduced plasma volume raises creatinine, urea, albumin, total protein, haematocrit and calcium together, generally by a modest proportion but sometimes substantially, and the pattern is recognisable precisely because so many analytes move in the same direction at once.

Persistent vomiting adds its own signature: hypokalaemia, hypochloraemia and a metabolic alkalosis, with magnesium frequently low alongside. That combination is a genuine finding requiring attention rather than an artefact, and distinguishing it from simple haemoconcentration is the reason a panel in this situation should include electrolytes and bicarbonate rather than being trimmed to the analytes of interest.

The practical rule the Journal has heard from every laboratory physician we have asked is to repeat rather than investigate: a panel drawn in a state of acute physiological disturbance, with several analytes moving coherently in one direction, is more informatively repeated after rehydration than pursued. Where the disturbance is itself the problem — where the vomiting is what needs addressing — the panel has already told you that, and it did not need a full workup to do it.

Thyroid function during energy restriction

Sustained energy restriction produces a characteristic and benign change in thyroid function tests: triiodothyronine falls, reverse triiodothyronine rises, thyroxine changes little and thyroid-stimulating hormone falls modestly or remains unchanged. This is the low-T3 pattern of adaptation to reduced energy availability, it is not hypothyroidism, and treating it as such is an error that predates this drug class by decades.

The relevant point for monitoring is that a thyroid panel drawn during rapid weight loss will frequently show a low or low-normal free T3, and that this does not indicate thyroid disease, does not require treatment, and reverses when energy balance is restored. Thyroid-stimulating hormone remains the appropriate first-line test for suspected thyroid dysfunction; adding free T3 to a panel during active weight loss reliably generates a result that requires explaining.

Separately and unrelatedly, this class carries a boxed warning in some jurisdictions derived from rodent thyroid C-cell findings. Serum calcitonin monitoring is not recommended for that purpose, and pharmacoepidemiological work examining thyroid cancer incidence in treated populations has not established the association the rodent data raised as a possibility.⁵ The Journal reports the boxed warning as what it is: a precaution derived from a rodent finding whose human relevance remains unestablished.

Analytical testing and clinical testing are different activities

A distinction has to be drawn firmly because the postbag suggests it frequently is not. The four independent testing services this market relies on — Janoshik, Medutest, PeptideMeter and VendorInvestigate — analyse material. They report chromatographic purity, identity by mass, peptide content where it is measured, and in the case of the verification services what could be established about a supplier. A clinical laboratory analyses a person. The two produce documents that superficially resemble each other and answer entirely unrelated questions.

A purity certificate reporting 99.1 per cent for a batch from WWB, CPC or QYB tells you nothing about anybody liver enzymes. A normal panel does not confirm that a vial contained what its label claimed, and an abnormal one does not establish that it did not. Where a person suspects a supply problem, the instrument for that is analytical testing of the material; where a person has an abnormal laboratory result, the instrument is clinical assessment. Substituting one for the other is a reliable way to spend money and learn nothing.

Compounds sold for research use only are not approved for human use in any jurisdiction, and nothing in this department should be read as guidance about using them or about monitoring their use.

How the Journal reports a laboratory figure

Five things accompany a laboratory number in these pages. The units, because international and conventional units differ for several analytes and the same value means different things in each. The reference interval used, with a note where the interval is contested, as it is for alanine aminotransferase. The baseline, because a change of 1.8 percentage points in HbA1c from a starting value of 8.3 is a different claim from the same change from 9.5. The estimand where the figure comes from a trial. And the reference change value where we are discussing an individual delta rather than a group mean.

We also state the assay method where it matters, which is more often than one would like: HbA1c in the presence of a haemoglobin variant, thyroid function in the presence of interfering antibodies, and creatinine measured by enzymatic against Jaffe methods all behave differently, and a comparison across methods is not a comparison.

This is a heavier apparatus than most publications carry and it exists because the alternative, in our experience, is a stream of technically accurate figures that lead readers to conclusions the data does not support. Errors in this apparatus should be reported to standards@compoundjournal.com; the correction log records what came of each one.

What this department is for

The Laboratory Notebook reports what tests measure, how they behave, and what has been found using them. It does not recommend monitoring schedules, interpret readers’ results, or advise on treatment. A laboratory result belongs in a conversation with a clinician who has the rest of the picture, and this publication is emphatically not that conversation.

Two standing notes. Several compounds discussed in these pages are sold for research use only and are not approved for human use in any jurisdiction; the Journal reports on them as commodities and as analytical problems, not as therapies. And where we describe what the pivotal trials monitored, that is reporting on trial protocols and not a template anybody should adopt from a magazine.

Correspondence is welcome at letters@compoundjournal.com. The Journal receives a steady flow of letters containing readers’ own panel results with a request for interpretation, and we do not provide it — not from caution but because a panel without a history, an examination and a reason for ordering it cannot be interpreted by anybody, including us.

Three artefacts recur often enough to be worth committing to memory. Creatinine falls because muscle mass falls, so estimated kidney function rises for a reason that has nothing to do with kidneys. Free triiodothyronine falls because energy intake fell, and that is adaptation rather than disease. And ferritin falls because inflammation falls, which may or may not coincide with iron stores falling. None of these is obscure and all three are routinely acted upon.