T3, T4, TSH Tests – Types, Indications, Contraindications

The thyroid gland produces hormones that are essential for normal body metabolism. Blood testing is now commonly available to determine the adequacy of the levels of thyroid hormones. These blood tests can define whether the thyroid gland’s hormone production is normal, overactive, or underactive.

The thyroid itself is regulated by another gland that is located in the brain, called the pituitary. In turn, the pituitary is regulated in part by the thyroid (via a “feedback” effect of thyroid hormone on the pituitary gland?) and by another gland called the hypothalamus.

The hypothalamus releases a hormone called thyrotropin-releasing hormone (TRH), which sends a signal to the pituitary to release thyroid-stimulating hormone (TSH). In turn, TSH sends a signal to the thyroid to release thyroid hormones. If a disruption occurs at any of these levels, a defect in thyroid hormone production may result in a deficiency of thyroid hormone (hypothyroidism).

Hypothalamus – TRH

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Pituitary – TSH

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Thyroid – T4 and T3

The rate of thyroid hormone production is controlled by the pituitary gland. If there is an insufficient amount of thyroid hormone circulating in the body to allow for normal functioning, the release of TSH is increased by the pituitary gland in an attempt to stimulate more thyroid hormone production

Thyroid function tests help to determine if your thyroid is not working correctly:

• hyperthyroid – an over-working thyroid
• hypothyroid – poor thyroid function

Symptoms of Hyperthyroid 

The symptoms of hyperthyroidism include the following

• fatigue or muscle weakness
• hand tremors
• Excessive sweating
• Heat intolerance
• Increased bowel movements
• Tremor? (usually fine shaking)
• mood swings
• increased appetite
• nervousness
• restlessness
• inability to concentrate
• irregular heartbeat
• difficulty sleeping
• fine, brittle hair, itching, hair loss
• nausea and vomiting
• breast development in men
• dizziness
• shortness of breath
• loss of consciousness?
• fast, irregular heart rate
• nervousness or anxiety
• rapid heartbeat
• skin dryness
• trouble sleeping
• weight loss
• increased frequency of bowel movements
• light periods or skipping periods.

Symptoms of Hypothyroidism

• Fatigue
• Weight gain or increased difficulty losing weight
• Coarse, dry hair
• Dry, rough pale skin ,Hair loss
• Cold intolerance (you can’t tolerate cold temperatures like those around you)
• Muscle cramps and frequent muscle aches
• Constipation
• Irritability
• Memory loss
• Abnormal menstrual cycles
• Decreased libido
• Increased sensitivity to cold
• Elevated blood cholesterol level
• Muscle aches, tenderness and stiffness
• Pain, stiffness or swelling in your joints
• Heavier than normal or irregular menstrual periods
• Thinning hair
• Slowed heart rate
• Depression
• Impaired memory

Test overall

Thyroid Hormones

Thyroid-stimulating hormone (TSH, thyrotropin)

Thyroid-stimulating hormone (TSH, thyrotropin) is generally increased in hypothyroidism and decreased in hyperthyroidism.

Its measurement is the most sensitive test for thyroid hormone function. TSH is produced in the pituitary gland. The production of TSH is controlled by thyrotropin-releasing hormone(TRH), which is produced in the hypothalamus. TSH levels may be suppressed by excess free T3 (fT3) or free T4 (fT4) in the blood.

Total thyroxine(Total T₄)

Total thyroxine is rarely measured, having been largely superseded by free thyroxine tests. Total thyroxine (Total T₄) is generally elevated in hyperthyroidism and decreased in hypothyroidism. It is usually slightly elevated in pregnancy secondary to increased levels of thyroid-binding globulin (TBG).

Total T4 is measured to see the bound and unbound levels of T4. The total T4 is less useful in cases where there could be protein abnormalities. The total T4 is less accurate due to a large amount of T4 that is bound. The total T3 is measured in clinical? practice since the T3 has decreased amount that is bound as compared to T4.

Reference ranges depend on the method of analysis. Results should always be interpreted using the range from the laboratory that performed the test. Example values are

Free thyroxine (fT₄)

Free thyroxine (fT₄) is generally elevated in hyperthyroidism and decreased in hypothyroidism.

Reference ranges depend on the method of analysis. Results should always be interpreted using the range from the laboratory that performed the test. Example values are

Total triiodothyronine (Total T₃) 

Total triiodothyronine (Total T₃) is rarely measured, having been largely superseded by free T3 tests. Total T3 is generally elevated in hyperthyroidism and decreased in hypothyroidism.

Reference ranges depend on the method of analysis. Results should always be interpreted using the range from the laboratory that performed the test. Example values are:

Free triiodothyronine(fT₃) 

Free triiodothyronine (fT₃) is generally elevated in hyperthyroidism and decreased in hypothyroidism.

Reference ranges depend on the method of analysis. Results should always be interpreted using the range from the laboratory that performed the test. Example values are:

Carrier proteins

Thyroxine-binding globulin

An increased thyroxine-binding globulin results in increased total thyroxine and total triiodothyronine without an actual increase in the hormonal activity of thyroid hormones.

Reference ranges:

Thyroglobulin

Reference ranges

Other binding hormones

• Transthyretin (prealbumin)
• Albumin

Protein binding function

Thyroid hormone uptake(T_uptake or T_{3 uptake}) 

Thyroid hormone uptake (T_uptake or T_{3 uptake}) is a measure of the unbound thyroxine-binding globulins in the blood, that is, the TBG that is unsaturated with thyroid hormone. Unsaturated TBG increases with decreased levels of thyroid hormones. It is not directly related to triiodothyronine, despite the name T_{3 uptake}.

Reference ranges

Other protein binding tests

• Thyroid Hormone Binding Ratio(THBR)
• Thyroxine-binding index (TBI)

References