TSH

La verità sul TSH e gli esami di laboratorio

Il dosaggio nel sangue del TSH (Thyroid Stimulating Hormone) è lo standard di riferimento su cui pressochè tutti i medici e gli specialisti endocrinologi oggi si basano per diagnosticare (od escludere)  un eventuale ipotiroidismo.  Se il TSH supera i valori massimi del laboratorio (in genere da 4 a 5 mU/l) il paziente è considerato ipotiroideo, ma in realtà se il valore è appena al di sopra del limite molti medici non lo considerano ancora un “vero” ipotiroidismo e consigliano di aspettare un ulteriore peggioramento prima di prendere una qualche iniziativa terapeutica. Se il TSH è invece nel range di normalità (generalmente da 0,5 a 4,5-5 mU/I) il paziente è considerato eutiroideo (cioè con una tiroide che funziona normalmente). Se il TSH è inferiore al minimo, il paziente viene giudicati ipertiroideo. E quello che è paradossale è che se il TSH è nel range, non vengono nemmeno più dosati FT3 ed FT4 (cioè le frazioni libere dei due ormoni tiroidei), quindi un TSH normale è tutto quello che oggi serve per escludere un ipotiroidismo.

Perchè il TSH  è spesso inaffidabile

Per decenni gli endocrinologi avevano cercato un esame di laboratorio che potesse individuare i casi di alterato funzionamento tiroideo in maniera semplice ed affidabile ma nessuno dei test proposti negli anni aveva mantenuto le aspettative ed era quindi stato rapidamente abbandonato. Nel 1973 un consesso di endocrinologi decise che  si era  finalmente trovato il test definitivo per diagnosticare l’ipotiroidismo, il TSH.  Va notato che per stabilire il range di normalità gli autori si erano basati sul valore del TSH in soli 200 volontari (apparentemente) sani. Inoltre all’inizio il range era assai più ampio che oggi arrivando come limite superiore fino a 15 mU/l, poi col passare degli anni questo limite fu ridotto a 10, poi a 5 e ora a 4-4,5mU/l.

Ma vediamo quali sono in sintesi i motivi per cui il TSH non può essere considerato un test sicuro ed affidabile per la diagnosi di ipotiroidismo:

  • Il TSH non è un ormone tiroideo ma ipofisario, non misura la funzione della tiroide, ma solo la risposta dell’ipofisi ai livelli di ormoni tiroidei presenti nel sangue
  • il range di normalità dell’esame del TSH è troppo ampio (è accettata come normale una variazione del suo valore di ben 10 volte!)
  • il TSH ha una vita media di poche ore e la sua produzione varia durante la giornata (per esempio nel pomeriggio è in genere più basso che al mattino)
  • il TSH può risultare normale o addirittura basso se è l’ipofisi ad avere problemi e quindi non riesce a rispondere al livello di ormoni circolanti (ipotiroidismo secondario), stessa cosa se il problema fosse nell’ipotalamo (ipotiroidismo terziario)
  • poichè il TSH risponde solo al livello di ormoni tiroidei circolanti esso sarà normale in caso di mancata conversione da T4 a T3, oppure di resistenza periferica agli ormoni, oppure di problemi nei recettori cellulari o nel trasporto degli ormoni all’interno della cellula. Cioè il fatto che T3 e T4 siano presenti nel sangue non significa necessariamente che poi questi  svolgano la loro azione nei tessuti.

Naturalmente in quest’ultimo caso anche la misurazione di FT3 e FT4 sarà inaffidabile per lo stesso motivo del TSH, gli ormoni in circolo ci sono (e quindi sono misurabili) ma per vari motivi essi non possono svolgere adeguatamente la loro funzione. E nei rari casi in cui oltre al TSH il medico prescriva un altro esame questo è quasi sempre solo l’ FT4. Ma il T4 come sappiamo è in realtà solo un precursore dell’ormone attivo T3 per cui misurare solo il T4 senza il T3 non ci consente di capire se abbiamo una buona conversione di un ormone nell’altro e se abbiamo quindi livelli accettabili di ormone attivo.

Infine c’è l’esperienza clinica, non solo nostra, che negli anni ci ha insegnato che ci sono molti, troppi casi di pazienti col TSH perfettamente nei limiti ma con tutta una serie di sintomi che indicano chiaramente che siamo di fronte ad un caso di ipotiroidismo, e la controprova ce l’abbiamo quando questi sintomi poi migliorano o scompaiono una volta che il paziente è sottoposto ad una adeguata terapia tiroidea.

E allora?

Quindi se il laboratorio non è sempre attendibile come può il medico fare una corretta diagnosi di ipotiroidismo? Semplicemente come facevano i medici fino a 40 anni fa quando questi test praticamente non esistevano, cioè basandosi essenzialmente sulla clinica (sintomi e segni del paziente) e magari aiutandosi con mezzi “empirici” come il test della temperatura basale di Barnes. E gli esami del sangue? Naturalmente è bene che vengano comunque prescritti e considerati ma i risultati andrebbero valutati in modo un pò più “elastico” basandosi sulla propria esperienza e capacità clinica e ricordando sempre che non siamo di fronte alle Tavole della Legge ma ad un  semplice test che molte volte, purtroppo, non ci può dire tutta la verità.

N.B. Gli esami degli anticorpi tiroidei anti-TPO-perossidasi (TPOAb) e anti-tireoglobulina (TgAb) sono invece piuttosto affidabili e ci consentono di sapere se siamo di fronte ad un problema autoimmunitario che interessa la tiroide (generalmente la tiroidite di Hashimoto che oggi è la più importante causa di ipotiroidismo).

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