Clinical Lab Diagnosis - Endocrine / Thyroid Lecture

As thyroid hormone levels increase in the blood, the pituitary gland produces _____ TSH and the thyroid produces _____ T4 and T3.

less; less

What produces TSH?

the pituitary gland

What produces T4 and T3?

the thyroid

AKA for Thyroid Stimulating Hormone

TSH / Thyrotropin

What does the pituitary create to stimulate the thyroid to produce T3 and T4?

TSH / Thyrotropin

AKA for T4

Thyroxine

Free T4

• Typically ordered with panel

• Tests measuring free T4
  more accurately reflect how the thyroid gland is functioning when checked with a TSH.

Total T4

Bound (TBG and albumin) and unbound T4

Can be affected by protein level and binding ability since most is bound

T4 interpretation

• High TSH and low FT4 or FT3 = primary hypothyroidism due to disease in the thyroid gland.

• Low TSH and low FT4 or FT3 = secondary hypothyroidism related to abnormal pituitary function

• Low TSH with an elevated FT4 or FT3 = hyperthyroidism

AKA for T3

Tri-iodothyronine

Free T3

Measurement of free T3 is possible but is often not reliable and therefore not typically helpful.

Total T3

Can be affected by protein level and binding ability since most is bound

T3 interpretation

• Patients who are hyperthyroid will have low TSH and an elevated T3 level. In some individuals with a low TSH, only the T3 is elevated and the FT4 or FTI is normal.

• T3 testing rarely is helpful in the hypothyroid patient, since it is the last test to become abnormal. Patients can be severely hypothyroid with a high TSH and low FT4 or FTI but have a normal T3.

Reverse T3

biologically inactive protein

some is produced normally in the body but is rapidly degraded

In healthy, non-hospitalized people, measurement of THIS does not help determine whether hypothyroidism exists or not, and is not clinically useful.

What are the thyroid antibodies?

TPO (Thyroid peroxidase) Antibody

TSHRAb (Thyroid stimulating hormone receptor antibodies)

Thyroid peroxidase (TPO) antibody

especially helpful in early Hashimoto thyroiditis when the TSH is elevated but the remaining thyroid is still able to maintain a normal free T4 level

What disease is TPO helpful in detecting?

Hashimoto’s Disease

Thyroid stimulating hormone receptor antibodies (TSHRAb)

Includes two types of autoantibodies that attach to proteins in the thyroid to which TSH normally binds (TSH receptors)

What are the two antibodies included in TSHRAb?

Thyroid stimulating immunoglobulin (TSI)

Thyroid binding inhibitory immunoglobulin (TBII)

Thyroid stimulating immunoglobulin (TSI)

Binds to receptors and promotes the production of thyroid hormones, leading to HYPERthyroidism.

Thyroid binding inhibitory immunoglobulin (TBII)

Blocks TSH from binding to receptors, blocking production of thyroid hormones and resulting in HYPOthyroidism.

Radioactive Iodine Uptake

Oral dose is Checked after 2, 6, 24 hours for uptake

A high uptake indicates that the thyroid gland is producing too much thyroxine

What is the most likely cause of the thyroid gland producing too much thyroxine?

Graves’ disease or hyper functioning nodules

What is the likely cause of hyperthyroidism combined with low Radioactive Iodine (RI) uptake?

thyroiditis or cancer