4 Thyroid Tests

Question 1

Best initial test of thyroid function?

Answer 1

TSH (Thyrotropin)

Stimulates thyroid to produce thyroid hormones (T4/T3)

Tightly regulated by serum levels of T4 and T3 (negative feedback)

Question 2

What is the difference between Free T4 and Total T4?

Answer 2

Free = readily available for use
Total = bound to proteins (storage)

Question 3

______ is diagnostically more relevant that total T4

Answer 3

Free T4 (free Thyroxine)

Total T4 extensively bound to plasma proteins
A small fraction circulates as free T4
Only free hormone is biologically active

Question 4

What is the usually ratio of T4 to T3 production?

Answer 4

20:1

Question 5

T4 is converted to…

Answer 5

T3 by LIVER (and thyroid and kidneys/other organs)

Question 6

Endogenous hyperthyroidism is due to …

Answer 6

Overproduction of thyroid hormones

Question 7

Examples of endogenous hyperthyroidism

Answer 7

Graves’ disease
Toxic multinodular goiter (TMG)
Toxic adenoma
Thyroiditis

Question 8

Examples of exogenous hyperthyroidism

Answer 8

Iatrogenic:
• Suppressive therapy (ie Thyroid cancer)
• Over-replacement in hypothyroidism

Factitious (surreptitious)

Question 9

Most common cause of hyperthyroidism?

Answer 9

Graves’ disease

W>M
More common before age 40
Predisposing factors - genetic susceptibility, infection, stress, smoking, pregnancy, iodine

Question 10

What is the pathophysiology of Graves’ disease?

Answer 10

Autoimmune

Antibodies bind to and stimulate thyroid stimulating hormone receptor —> increased thyroid hormone production

Question 11

What physical findings points to Graves’ disease in a hyperthyroid patient?

Answer 11

Ophthalmopathy (exophthalmos, proptosis)

Dermopathy of shins

Firm, diffuse goiter

Question 12

What physical findings points to TMG in a hyperthyroid patient?

Answer 12

Firm heterogenous goiter of variable size

Retrosternal extension

Question 13

What physical findings points to toxic adenoma in a hyperthyroid patient?

Answer 13

Palpable nodule

Question 14

What physical findings points to thyroiditis in a hyperthyroid patient?

Answer 14

Exquisitely tender thyroid

Question 15

What thyroid antibodies should we order for a hyperthyroid patient?

Answer 15

TRAb (TSH Receptor Antibodies)

TSI (Thyroid-stimulating immunoglobulin)

TBII (Thyroid-binding inhibitory immunoglobulin)

Question 16

Would a thyroid uptake and scan be helpful to determine the cause of a Grave’s patient’s hyperthyroidism?

Answer 16

May not provide much additional information if the diagnosis has already been made clinically and biochemically.

But sure, go ahead

Question 17

Radioactive iodine uptake and scan is used to evaluate…

Answer 17

Hyperthyroidism

Patient swallow radioactive iodine capsule and gamma probe measures uptake at 6 and 24 hours

Normal = 15-25% uptake

Question 18

Uptake and scan results

Answer 18

High uptake indicates excessive synthesis

Low indicates either inflammation/destruction of thyroid tissue OR extrathyroidal source of thyroid hormone

Question 19

How will graves look on uptake and scan?

Answer 19

Homogenous elevated uptake

Question 20

How will nodules or TMG appear on uptake and scan?

Answer 20

Irregular uptake

Hyper functioning “hot” nodules - rarely malignant, no need for biopsy

Question 21

Treatment for Graves’ disease?

Answer 21

BETA BLOCKERS for symptom control

Antithyroid drugs
• Methimazole
• Propylthiouracil (PTU)

Radioactive iodine ablation

Surgery?

Question 22

Most common cause of primary hypothyroidism?

Answer 22

Hashimoto thyroiditis

Autoimmune

Question 23

What antibodies do you anticipate being present in Hashimoto’s?

Answer 23

TgAb

TPO antibodies

MAYBE TRAb (TBII) - but that’s more likely in Graves

Question 24

Central hypothyroidism makes up ______ of cases of hypothyroidism

Answer 24

<5%

Can be pituitary (secondary) or hypothalamic (tertiary)

Will have low serum thyroid hormones AND low TSH

Question 25

What is your next step if you suspect central hypothyroidism?

Answer 25

Pituitary MRI

Question 26

Iatrogenic causes of hypothyroidism

Answer 26

Treatment with radioactive iodine

Medications
• Lithium
• Amiodarone (can also cause hyper)
• Other iodine-containing drugs
• Contrast agents

Question 27

What are your first steps if you notice a thyroid nodule on neck exam?

Answer 27

TSH and ultrasound

Question 28

What are the chances that a thyroid nodule is malignant

Answer 28

Not likely

Nodules are common and thyroid cancer makes up 4-6.5% of nodules

Question 29

What does a thyroid U/S tell us?

Answer 29

Evaluates thyroid size and morphology

Most sensitive test for evaluating nodules

Can detect lesions of 2-3 mm

Helps guide FNA biopsy

Question 30

Limitations of thyroid ultrasound

Answer 30

Observer dependent

Unable to “see” retrosternal lesions

Can’t unequivocally differentiate between benign and malignant nodules

Biopsy needed to confirm dx

Question 31

“Cold” nodules on thyroid uptake and scan

Answer 31

Hypofunctioning nodules

More likely to be malignant (Cold=Cancer)

Question 32

“Hot” nodules on thyroid uptake and scan

Answer 32

Hyper functioning nodules

Less likely to be malignant

Question 33

How to evaluate a thyroid nodule

Answer 33

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

Question 34

Single most accurate, reliable, cost-effective test to diagnose thyroid cancer

Answer 34

Fine needle aspiration biopsy

Overall diagnostic accuracy exceeds 95%