4 Thyroid Tests Flashcards by Hannah Frank

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)

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What is the difference between Free T4 and Total T4?

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

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______ 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

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What is the usually ratio of T4 to T3 production?

20:1

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T4 is converted to…

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

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Endogenous hyperthyroidism is due to …

Overproduction of thyroid hormones

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Examples of endogenous hyperthyroidism

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

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Examples of exogenous hyperthyroidism

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

Factitious (surreptitious)

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Most common cause of hyperthyroidism?

Graves’ disease

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

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What is the pathophysiology of Graves’ disease?

Autoimmune

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

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

Ophthalmopathy (exophthalmos, proptosis)

Dermopathy of shins

Firm, diffuse goiter

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What physical findings points to TMG in a hyperthyroid patient?

Firm heterogenous goiter of variable size

Retrosternal extension

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

Palpable nodule

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What physical findings points to thyroiditis in a hyperthyroid patient?

Exquisitely tender thyroid

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What thyroid antibodies should we order for a hyperthyroid patient?

TRAb (TSH Receptor Antibodies)

TSI (Thyroid-stimulating immunoglobulin)

TBII (Thyroid-binding inhibitory immunoglobulin)

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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%