4 Thyroid Tests Flashcards
Best initial test of thyroid function?
TSH (Thyrotropin)
Stimulates thyroid to produce thyroid hormones (T4/T3)
Tightly regulated by serum levels of T4 and T3 (negative feedback)
What is the difference between Free T4 and Total T4?
Free = readily available for use Total = bound to proteins (storage)
______ is diagnostically more relevant that total T4
Free T4 (free Thyroxine)
Total T4 extensively bound to plasma proteins
A small fraction circulates as free T4
Only free hormone is biologically active
What is the usually ratio of T4 to T3 production?
20:1
T4 is converted to…
T3 by LIVER (and thyroid and kidneys/other organs)
Endogenous hyperthyroidism is due to …
Overproduction of thyroid hormones
Examples of endogenous hyperthyroidism
Graves’ disease
Toxic multinodular goiter (TMG)
Toxic adenoma
Thyroiditis
Examples of exogenous hyperthyroidism
Iatrogenic:
• Suppressive therapy (ie Thyroid cancer)
• Over-replacement in hypothyroidism
Factitious (surreptitious)
Most common cause of hyperthyroidism?
Graves’ disease
W>M
More common before age 40
Predisposing factors - genetic susceptibility, infection, stress, smoking, pregnancy, iodine
What is the pathophysiology of Graves’ disease?
Autoimmune
Antibodies bind to and stimulate thyroid stimulating hormone receptor —> increased thyroid hormone production
What physical findings points to Graves’ disease in a hyperthyroid patient?
Ophthalmopathy (exophthalmos, proptosis)
Dermopathy of shins
Firm, diffuse goiter
What physical findings points to TMG in a hyperthyroid patient?
Firm heterogenous goiter of variable size
Retrosternal extension
What physical findings points to toxic adenoma in a hyperthyroid patient?
Palpable nodule
What physical findings points to thyroiditis in a hyperthyroid patient?
Exquisitely tender thyroid
What thyroid antibodies should we order for a hyperthyroid patient?
TRAb (TSH Receptor Antibodies)
TSI (Thyroid-stimulating immunoglobulin)
TBII (Thyroid-binding inhibitory immunoglobulin)
Would a thyroid uptake and scan be helpful to determine the cause of a Grave’s patient’s hyperthyroidism?
May not provide much additional information if the diagnosis has already been made clinically and biochemically.
But sure, go ahead
Radioactive iodine uptake and scan is used to evaluate…
Hyperthyroidism
Patient swallow radioactive iodine capsule and gamma probe measures uptake at 6 and 24 hours
Normal = 15-25% uptake
Uptake and scan results
High uptake indicates excessive synthesis
Low indicates either inflammation/destruction of thyroid tissue OR extrathyroidal source of thyroid hormone
How will graves look on uptake and scan?
Homogenous elevated uptake
How will nodules or TMG appear on uptake and scan?
Irregular uptake
Hyper functioning “hot” nodules - rarely malignant, no need for biopsy
Treatment for Graves’ disease?
BETA BLOCKERS for symptom control
Antithyroid drugs
• Methimazole
• Propylthiouracil (PTU)
Radioactive iodine ablation
Surgery?
Most common cause of primary hypothyroidism?
Hashimoto thyroiditis
Autoimmune
What antibodies do you anticipate being present in Hashimoto’s?
TgAb
TPO antibodies
MAYBE TRAb (TBII) - but that’s more likely in Graves
Central hypothyroidism makes up ______ of cases of hypothyroidism
<5%
Can be pituitary (secondary) or hypothalamic (tertiary)
Will have low serum thyroid hormones AND low TSH
What is your next step if you suspect central hypothyroidism?
Pituitary MRI
Iatrogenic causes of hypothyroidism
Treatment with radioactive iodine
Medications • Lithium • Amiodarone (can also cause hyper) • Other iodine-containing drugs • Contrast agents
What are your first steps if you notice a thyroid nodule on neck exam?
TSH and ultrasound
What are the chances that a thyroid nodule is malignant
Not likely
Nodules are common and thyroid cancer makes up 4-6.5% of nodules
What does a thyroid U/S tell us?
Evaluates thyroid size and morphology
Most sensitive test for evaluating nodules
Can detect lesions of 2-3 mm
Helps guide FNA biopsy
Limitations of thyroid ultrasound
Observer dependent
Unable to “see” retrosternal lesions
Can’t unequivocally differentiate between benign and malignant nodules
Biopsy needed to confirm dx
“Cold” nodules on thyroid uptake and scan
Hypofunctioning nodules
More likely to be malignant (Cold=Cancer)
“Hot” nodules on thyroid uptake and scan
Hyper functioning nodules
Less likely to be malignant
How to evaluate a thyroid nodule
Order TSH and U/S
If TSH low —> order thyroid uptake and scan
Hot nodule —> likely benign but check FT4 and if high treat hyperthyroidism
Cold nodule —> consider FNA biopsy
If TSH normal/elevated —> check for TPO antibodies, if high treat for hashimoto
Consider FNA
Single most accurate, reliable, cost-effective test to diagnose thyroid cancer
Fine needle aspiration biopsy
Overall diagnostic accuracy exceeds 95%
