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E22 Endocrinology
Thyroid Toronto Notes 2019
• itincreasessensitivitytocatecholamines,byup-regulatingtheirreceptors,butdoesnotaltertheirblood concentrations
• thyroidhormoneisrequiredfornormalgrowthinthefetusandchild,includingthecentralnervous system, via stimulation of GH release, in synergism with cortisol
Regulation of Thyroid Function
• extrathyroid
■ stimulation of thyroid by TSH, epinephrine, prostaglandins (cAMP stimulators)
■ T3 negatively feeds back on anterior pituitary to inhibit TSH and on hypothalamus to inhibit TRH
• intrathyroid(autoregulation)
■ synthesis (Wolff-Chaikoff effect, Jod-Basedow effect)
■ there is varying thyroid sensitivity to TSH in response to iodide availability
■ increased ratio of T3 to T4 in iodide deficiency
■ increased activity of peripheral 5’ deiodinase in hypothyroidism increases T3 production despite low
T4 levels
Tests of Thyroid Function and Structure
TSH
• sensitiveTSH(sTSH)isthebesttestforassessingthyroidfunction • hyperthyroidism
■ primary: TSH is low because of negative feedback from increased levels of circulating T3 and T4
■ secondary: increased TSH results in increased T3 and T4 • hypothyroidism
■ primary: increased TSH (most sensitive test) because of less negative feedback from T3 and T4
■ secondary: TSH is low or normal with variable response to TRH depending on the site of the lesion
(pituitary or hypothalamic)
Free T3 and Free T4
• standard assessment of thyroid function measures TSH and, if necessary, free T4. Free T3 should only be measured in the small subset of patients with hyperthyroidism and suspected T3 toxicosis. TSH would be suppressed, free T4 normal, and free T3 elevated
Thyroid Autoantibodies
• thyroglobulinantibodies(TgAb),anti-thyroidperoxidaseantibodies(TPOAb),andTSHreceptor antibodies (TRAb) of the blocking variety are increased in Hashimoto’s disease; normal variant in 10- 20% of individuals
• TRAb of the stimulating variety are also referred to as thyroid stimulating immunoglobulins (TSI) and cause Graves’ disease. However, both TRAb receptor blocking and stimulating antibodies are seen in patients with Graves’ disease
Plasma Thyroglobulin
• usedtomonitorforresidualthyroidtissuepost-thyroidectomy,e.g.tumourmarkerforthyroidcancer recurrence
• normalorelevatedlevelsmaysuggestpersistent,recurrent,ormetastaticdisease
Serum Calcitonin
• notroutinelydonetoinvestigatethyroidnodules
• orderedifsuspicionofmedullarythyroidcarcinomaorfamilyhistoryofMENIIaorIIbsyndromes
■ used to monitor for residual or recurrent medullary thyroid cancer
Thyroid Imaging/Scans
• normalglandsize15-20g(estimatedbypalpation)
• thyroidU/S
■ to measure size of gland, characterize thyroid nodules, facilitate fine needle aspirate biopsy (FNAB)
• radioisotopethyroidscan(Technetium-99)
• testofstructure:orderifthereisathyroidnoduleandpatientishyperthyroidwithlowTSH
■ differentiates between hot (functioning → excess thyroid hormone production) and cold (non- functioning) nodules
• hotnodule→verylowchancemalignancy;treathyperthyroidism
• coldnodule→~5-15%chancemalignancy;furtherworkuprequired(U/SandFNAB)
• radioactiveiodineuptake(RAIU)
■ test of function: order if patient is thyrotoxic
■ RAIU measures the turnover of iodine by thyroid gland in vivo
■ if ↑ uptake (i.e. incorporated), gland is overactive (hyperthyroid)
■ if ↑ uptake (i.e. not incorporated), gland is leaking thyroid hormone (e.g. thyroiditis), exogenous
thyroid hormone use, or excess iodine intake (e.g. amiodarone or contrast dye, which has high
iodine content)
• seeFigure9,ApproachtotheEvaluationofaThyroidNodule,E21forfurtherinformationregardingthe
Thyroid Assessment
• Serum thyroid hormones (TSH, T3, T4)
• Antibodies (TRAb, TgAb and TPOAb)
• Thyroglobulin (to monitor thyroid cancer)
• Thyroid ultrasound
• Nuclear uptake and scan (for
hyperthyroidism)
• Biopsy (FNA)
Does this Patient have a Goitre?
From The Rational Clinical Examination
JAMA 2009; http://www.jamaevidence.com/ content/3480618
Study: Systematic review of articles assessing the accuracy and precision of the clinical exam in the diagnosis of a goitre.
Results: Clinical diagnosis was based on degree of lateral prominence, visibility, and palpability of the thyroid gland. No evidence exists to support the superiority of any one method.
The combined results of 4 studies detail the predictive utility of assessing grades of thyroid gland weight:
Weight Reference LR+ 95% CI
0-20 g normal 20-40 g 1-2x >40 g >2x
0.15 (0.10-0.21) 1.9 (1.1-3.0) 25.0 (2.6-175)
Alternatively, defining a goitre as mass larger than the distal phalanx of the thumb has been shown to have an LR+ of 3.0 (95% CI 2.5-3.5) and LR- of 0.30 (95% CI 0.24-0.37) in children, and an LR+ of 4.7 (95% CI 3.6-6.0) and LR- of 0.08 (95% CI 0.02-0.27) for the presence of a goitre.
Conclusions: Use of weight of thyroid tissue is an appropriate method of diagnosing a goitre, while comparing the size of thyroid mass to the distal phalanx of the thumb may be a useful alternative.
utility of these scans