Free Thyroxine - All You Need To Know About fT4

Free Thyroxine or (fT4) is one of the two hormones produced by the thyroid gland. Mostly thyroid produces hormones in the form of fT4. Since this hormone is relatively inactive, it is converted into the much more active T3 in the liver and other tissues. Together with TSH, fT4 gives clues on how your thyroid is functioning. So, if you are looking to get a full picture of your thyroid health and what it means for your fertility, it is necessary to test T4 along with your regular thyroid work-up. 

What Is fT4?

Thyroxine (T4) is an important hormone secreted by thyroid into the bloodstream. T4 is generally of 2 forms: Free T4 and bound T4. Free T4 (fT4) circulates freely through the tissues whereas bound T4 is found attached to the proteins, thereby, preventing it from entering the tissues. Over 99% of T4 are said to be in bound form ¹. Since the free form is a biologically active form that is made more available for the tissues to perform its stipulated functions, accuracy can be achieved with fT4 than testing the bound T4.

Role Of fT4

When thyroid hormone levels decrease in the blood, the pituitary gland produces TSH. TSH in turn stimulates the thyroid gland to produce and release T4 and T3 by binding to the TSH receptor. When thyroid hormone levels increase in the blood, the pituitary gland produces less TSH, and thyroid produces less T4 and T3. This hormonal regulation is termed as Negative Feedback Mechanism. 

Also, T4 is secreted in its inactive form at first which gets converted to an active form called  triiodothyronine (T3) to facilitate its functions which include maintaining the metabolic rate, muscle control, cardiac and digestive functions, brain development and bone maintenance.² This is made possible with the help of organs such as the liver and kidney.

Understanding Thyroid Disorders

T4 levels along with TSH levels and sometimes T3 levels are used to evaluate your thyroid function. These levels are useful to analyze thyroid disorders namely- hypothyroidism and hyperthyroidism. 

Fact: 

If your TSH levels are found to be higher than normal, it may indicate poor thyroid function wherein the gland is not producing enough of the thyroid hormones (T4 & T3). This condition is known as Hypothyroidism. In contradiction, If your thyroid levels are found to be lower than normal, it may indicate over production of the thyroid hormones known as Hyperthyroidism.

These disorders may impact fertility by ³: 

• Disrupting the menstrual cycle making it difficult to conceive 
• Interfering with the release of eggs from the ovary (ovulation)
• Increasing the risk of early-term miscarriage
• Increasing the risk for premature birth

Implication: 

Both hyper and hypothyroidism may negatively impact fertility by affecting both - the  ability to get pregnant as well as carry a child to term ³. Should your results indicate a thyroid disorder, consult your doctor and take corrective measures.

When Can I Do An fT4 Test? 

Your fT4 test does not require any special preparations. It can be performed anytime during the day. 

What Can Differing fT4 Levels Tell You?

1. Normal Levels

A normal reference range for fT4 is somewhere between 0.8-2.7 ng/dL.  

2. High Levels

High levels of fT4 may indicate an overactive thyroid, also known as hyperthyroidism. Symptoms of hyperthyroidism include: ⁴

• Anxiety
• Weight loss
• Tremors in the hands
• Increased heart rate
• Puffiness
• Bulging of the eyes
• Trouble sleeping

3. Low Levels 

Low levels of fT4 may indicate an underactive thyroid, also known as hypothyroidism.  Symptoms of hypothyroidism include: ⁴

• Weight gain 
• Fatigue
• Hair loss
• Low tolerance for cold temperatures
• Irregular menstrual periods
• Constipation