Free T4 (Thyroxine) Lab Values

Hey everyone, Abby here with NURSING.com. In this video, we’ll discuss a lab value for free T4 or thyroxine. It’s a thyroid hormone. Let’s get started.

Free T4 or thyroxine. This lab measures the amount of T4 hormone that is free and unbound. When it’s free and unbound, the thyroid hormone is actually able to affect the body tissues. Thyroid is located here in the throat and it’s that butterfly-shaped gland that produces T4 and T3. When talking about hormones, it’s really important to talk about the feedback loop. Most notably, the thyroid hormone, negative feedback loop. Now, TRH or thyrotropin-releasing hormone is produced by the hypothalamus in the brain. The hypothalamus releases the TRH and it stimulates release of thyroid-stimulating hormone or TSH from the anterior pituitary gland. TSH then travels to the thyroid and the thyroid responds by producing T3 and T4 thyroid hormones. When T3 and T4 are in systemic circulation, there’s a feedback to the hypothalamus that inhibits the release of more TRH. If TRH is inhibited, then TSH is not stimulated to be released by the anterior pituitary gland and therefore the thyroid isn’t stimulated to produce thyroid hormones. It’s all about balance when there’s enough T3 and T4 in the blood, TRH doesn’t need to be released. When these levels go down, the process starts over again. The hypothalamus detects that there isn’t enough, therefore it releases TRH which releases TSH, which goes to the thyroid to produce T3 and T4. 

Now there’s a difference between T4 testing and free T4 testing. In this module, we’re speaking about free T4 testing. This is remember, free and unbound, unbound in the free T4. Whereas T4 testing measures both. Why that’s helpful is that we can determine the severity of hyper or hypothyroidism when combined with a TSH lab, because sometimes the TSH will actually appear normal. Additionally, it will be used with a TSH lab to determine if the disease is suspected to be in the hypothalamus or the pituitary gland and not the thyroid itself.

Normal therapeutic values for free T4 are 0.8 to 1.8 nanograms per deciliter. This lab is collected in a serum separator tube, or a gold top. Now what do we know about hyper versus hypothyroidism? Can you think about some clinical presentations of what an individual with too much thyroid hormone might look like? You were right! If the lab value of free T4 is increased, that’s going to indicate hyperthyroidism. Remember, those with hyperthyroidism are hyper and fast. Exophthalmos can even develop. This is kind of like how your body would respond in a fight or flight response. So, your heart rate is going to increase, you’re gonna be sweating, heat intolerant, a very high metabolic rate. Whereas if the free T4 value is decreased, that indicates hypothyroidism. Individuals with hypothyroidism, we say, tend to be low and slow. You might even see a goiter formation really, depending on either one. It could be hyper or hypothyroidism. 

Now, linchpins for this lesson, the thyroxine, which is T4, this lab is the free and unbound version of T4 available to affect the body tissues. It’s normal at 0.8 to 1.8 nanograms per deciliter, when it’s high it’s hyper, when it’s low it’s hypothyroidism. And remember that negative feedback loop, it’s all about balance. Great job on this lesson and this wraps it up. Now, go out and be your best self today and as always, happy nursing!

References:

• https://www.uptodate.com/contents/laboratory-assessment-of-thyroid-function?
• https://www.uptodate.com/contents/laboratory-assessment-of-thyroid-function?search=free%20t4%20lab%20value%20ranges&topicRef=114890&source=related_link
• https://www.uptodate.com/contents/high-free-t4-in-adults?search=free%20t4%20high&source=panel_search_result&selectedTitle=2~150&usage_type=panel&display_rank=1