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Endocrine & MetabolicThyroid

Thyroid Function Test

Updated: 10th Nov 2023

Physiology

·      =    [Hypothalamus] > TRH > [Anterior pituitary] > TSH > [Follicular cells] > T4 & T3 > [Tissue]

[Hypothyroidism] – Starting= thyroxine treatment

·      =    < 60y & no IHD – Full dosage [1.6 mcg/kg] e.g. 75kg man =3D 125m= cg

·      =    Else – Low dose [25-50mcg]

·      =    Test TSH @ 6-8w

[Hypothyroidism] – Monitori= ng thyroxine treatment

·      =    Test TSH @ 6-8w after dose change

·      =    Alter dosage by 25mcg if needed

·      =    TSH aim

·      =    lower half of reference range [0.4-2.5]

·      =    below reference range acceptable in young patents on high dose Rx.=

·      =    Do not overtreat [T4 high or towards upper limit, TSH suppressed]

·      =    Once established, monitor anually [25% poor compliance or absorption] <= o:p>

[Hypothyroidism] – Stopping= thyroxine treatment

·      =    Only if transient cause suspected – stop after 6/12, recheck TFT in 6w<= o:p>

Tests

·      =    T3 & T4 – [Free form active] [T3 5 times more potent, 1/2 life 24h] [T4 1/2 li= ve 7d]

·      =    Thyroglobulin autoantibodies – [High – Graves, Hashimoto’s, De Quervains, 7% of M & 15-20% of F without disease]

·  &n= bsp;      Anti-TPO antibodies – [High – Hashimoto’s, idiopathic atrophy] [Low – Graves, De Quervains,= 8% M & 10% F withouth disease]

Hypothyroidism

·      =    Worldwide – [Iodine deficiency most common]

·      =    Developed nations – [Hashimotos – Autoimmune, +- Goiter, high anti-TPO antibody, low thyroglobin antibody], Chronic autoimmune thyroiditis

·  &n= bsp;      Other – Iatrogenic [Anti-thyroid Rx, Lithium, Amiodarone], De Quervain’= s, Silent thyroiditis (typically post partum)

·  &n= bsp;      Subclinic hypothyroidism [Raised TSH, Normal T4, asymptomatic]=

·  &n= bsp;      Repeat in 3-6/12 with anti-TPO antibodies

·  &n= bsp;      TSH =3D< 10

·  &n= bsp;      2-5% progression to overt hypothyroidism per annum [80% overt by 4y if anti-TPO positive]

·  &n= bsp;      Consider Rx. if TSH rising, Goiter or trial if symptoms for 3-6/12

·  &n= bsp;      Else, yearly surveillance of TSH if anti-TPO positive, 3 yearly if anti= -TPO negative

·  &n= bsp;      If TSH > 10

·      =    Treat following confirmation [as high risk of progression]

Hyperthyroidism

·      =    90% – [Graves – Thyroid stimulating IgG against TSH receptors & periorbital tissue] [Toxic nodular – single or multiple]<= /p>

·      =    10% – Thyroiditis [De Quervain’s, Silent/post partum, Amiodarone] =

Referral criteria

·      =    Hypothyroidism – [<16] [pregnant or post partum] [pituitary disease] [Newborn] – consider if [active/unstable IHD] [rx. with amiodarone or lithi= um] [non responsive symptoms with adequate Rx.] [TSH persistently raised on Rx.= ]

·      =    Hyperthyroidism – [All]

Patient information

·      =    Hypothyroidsm – http://www.patient.co.uk/health/hypothyroidism-underac= tive-thyroid-leaflet

·      =    Advice for newly diagnosed hypothyroidism=

·      =    It will take at least a week after initiating therapy for the symptoms = to improve.  In those with muscle weakness, stiffness, or cognitive defects that it may take up to six months= to fully resolve the symptoms.

·      =    There will be no noticeable effect if one dose is missed

·      =    Take a double dose on the day after a missed tablet (except in active ischaemic heart disease and atrial fibrillation)

·      =    Levothyroxine should be taken on an empty stomach to maximise absorptio= n

·      =    Treatment is life long.

·      =    When dose is adjusted, you will need a repeat blood test in 6 to 8 weeks.  When dose the right do= se is found, you will need yearly measurement of TSH level -the dose may be adjus= ted accordingly. 

·      =    In the UK, patients with hypothyroidism are eligible for a medical exemption certificate for prescription charges