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Mechanism 02 of 16

Ion mimicry and active transport

No lookalike chemistry is required — the tracer simply is an ion the pump cannot distinguish from its substrate. All the selectivity comes from where that pump is expressed.

The instructive pair here is iodide versus pertechnetate. Both are trapped by the same symporter. Only iodide is then organified onto thyroglobulin, and that second step is the entire reason iodide is still in the thyroid at 24 hours while pertechnetate, whose thyroid uptake peaks at about 20 minutes, then washes back out. A nodule that is hot on pertechnetate but cold on iodine — the discordant nodule — is exactly this trap-without-organification mismatch, and it is the reason a pertechnetate-hot nodule cannot on its own be called benign.

The cardiac members of the family, thallium and rubidium, are potassium analogues pumped in by Na⁺/K⁺-ATPase. Thallium’s signature is redistribution: because the exchange with the blood pool never stops, ischaemic but viable myocardium fills in on delayed images while scar does not.

Blood · basolateral surfaceThyrocyte cytosolNIS2 Na⁺ : 1 I⁻ symportthyroid peroxidasecovalent bond to thyroglobulinNa⁺/K⁺-ATPaseTl⁺ and Rb⁺ read as K⁺redistributionover hoursno organification→ image at 20 minutesI⁻T-gI⁻T-gI⁻T-gTcO₄TcO₄Tl⁺Tl⁺
iodide / thalliumorganified onto thyroglobulinpertechnetate — washes back out
Same door, different fate. The symporter cannot tell iodide from pertechnetate — both are trapped. Thyroid peroxidase then binds iodide covalently onto thyroglobulin, and it stays. Pertechnetate has no such step, so it turns around and leaves: image it at 20 minutes, not 24 hours. Next door, Na⁺/K⁺-ATPase does the same trick with thallium as a potassium analogue.

The agents

I-123 sodium iodide—

SPECT / planar

I-123 · t½ 13.2 h · 159 keV γ — near-ideal for a gamma camera

BloodThyrocyteNIS2 Na⁺ : 1 I⁻thyroid peroxidaseorganified onto thyroglobulin→ still there at 24 hI⁻T-gI⁻T-gI⁻T-g
Handle
Sodium-iodide symporter (NIS), then thyroid peroxidase
Trapping
Trapped by NIS at the basolateral membrane (driven by Na⁺/K⁺-ATPase), then organified onto thyroglobulin tyrosines. The second step is what gives 24-hour retention.
Use
Thyroid uptake and scan, whole-body scan for differentiated thyroid cancer, ectopic thyroid.
Pitfall
Recent iodinated contrast, amiodarone and exogenous thyroid hormone all suppress uptake — always ask about a recent CT. NIS is also expressed in salivary glands, gastric mucosa, choroid plexus and lactating breast, which is normal uptake on a whole-body scan and a classic misread.

I-131 sodium iodide—

Therapy

I-131 · t½ 8.02 d · 364 keV γ (poor imaging, high scatter) · β⁻ 606 keV max

BloodThyroid follicleNISTPOfollicleβ⁻ 606 keV max · path ~0.8 mmsame targeting,therapeutic payloadI⁻T-gI⁻T-gI⁻T-g
Handle
Same NIS and organification pathway
Trapping
Identical targeting; the β⁻ particle, with a path length of ~0.8 mm, delivers the therapeutic dose to the follicle.
Use
Hyperthyroidism, remnant ablation and treatment of metastatic differentiated thyroid cancer.
Pitfall
Stunning after a high pre-therapy diagnostic dose. Requires TSH stimulation (withdrawal or recombinant TSH) and a low-iodine diet. Sialadenitis, xerostomia and a small secondary-malignancy risk with cumulative activity.

Tc-99m pertechnetateTcO₄⁻

SPECT / planar

Tc-99m · t½ 6.01 h · 140 keV γ · from a Mo-99/Tc-99m generator

BloodThyrocyteNISTPOno organification step→ image at 20 minutes, not 24 hourstrapped exactlylike iodide…TcO₄TcO₄
Handle
NIS — trapped but not organified
Trapping
Similar charge and ionic radius to iodide, so NIS takes it up. With no peroxidase step it washes straight back out, which sets the 20-minute imaging window.
Use
Thyroid scan, Meckel diverticulum (ectopic gastric mucosa), salivary and lacrimal studies, first-pass and brain-death flow, RBC labelling.
Pitfall
The discordant nodule — hot on pertechnetate, cold on iodine — occurs in a few per cent of nodules, so a pertechnetate-hot nodule cannot be called benign without I-123. Physiologic salivary, gastric and choroid plexus activity. For a Meckel scan, pentagastrin or an H₂ blocker improves sensitivity; recent barium can mask a focus (false negative), while laxatives, enemas and other mucosal irritants cause false positives.

Tl-201 thallous chloride—

SPECT / planar

Tl-201 · t½ 73 h · 69–81 keV mercury characteristic X-rays — low energy, high dose

BloodMyocyteNa⁺/K⁺-ATPaseTl⁺ is read as K⁺ — extraction ~85%continuous exchangewith the blood poolredistribution:viable ischaemic myocardium fills inTl⁺Tl⁺Tl⁺
Handle
Na⁺/K⁺-ATPase (potassium analogue)
Trapping
Actively pumped into viable myocytes; first-pass extraction ~85%. Initial distribution follows flow, but exchange with the blood pool continues, producing redistribution.
Use
Myocardial perfusion and viability (largely superseded), parathyroid and tumour viability historically.
Pitfall
High radiation dose and poor spatial resolution from the low photon energy. Marked soft-tissue attenuation. Long protocol. Reverse redistribution is a recognised and poorly specific finding.

Rb-82 chlorideCardioGen-82

PET

Rb-82 · t½ 76 s · from a Sr-82/Rb-82 generator — no cyclotron needed

BloodMyocyteNa⁺/K⁺-ATPaset½ 76 sdecays where it landsuptakeflowextraction rolls offat hyperaemic flowRb⁺Rb⁺Rb⁺
Handle
Na⁺/K⁺-ATPase (potassium analogue)
Trapping
Same pump as thallium, imaged as a positron emitter. First-pass extraction ~65% at rest, falling steeply at hyperaemic flow.
Use
PET myocardial perfusion with absolute flow quantification; generator-based, so it suits centres without a cyclotron.
Pitfall
The 76-second half-life demands precise infusion and acquisition timing. Daily Sr-82/Sr-85 breakthrough testing is mandatory. Long positron range degrades resolution, and the short half-life rules out treadmill exercise.
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