Thyroid Testing Beyond TSH

The standard thyroid check is a single number, TSH. For a lot of people that is the only thyroid test they have ever had. It is a good first screen, but it measures the signal asking for hormone, not the hormone itself, and some questions need the hormone itself.

What TSH actually is

The thyroid does not run itself. The pituitary gland at the base of the brain reads how much thyroid hormone is circulating and sends out TSH, thyroid stimulating hormone, to ask for more or less. A high TSH means the pituitary is shouting because hormone is low. A low TSH means it has gone quiet because hormone is high. So TSH is a report on what the pituitary thinks, one step removed from the hormone itself.

The thyroid mostly makes T4, which is a storage form. The active hormone is T3, and roughly 80 percent of it is made outside the thyroid, when tissues such as the liver, kidney and muscle strip one iodine atom off T4. That conversion step is one place a TSH only check gives an incomplete picture.

The tests that fill in the picture

• Free T4 is the storage hormone actually available to tissues, not bound up by proteins in the blood.
• Free T3 is the active hormone available to tissues. A normal TSH with a free T3 sitting at the bottom of the range usually means the gland is fine and conversion has slowed. Prolonged calorie restriction, illness, heavy training stress and some medications can slow it, and the answer is usually fixing the cause rather than adding hormone.
• Thyroid peroxidase antibodies show whether the immune system is attacking the gland. Autoimmune thyroiditis is the most common cause of an underactive thyroid in the United States, and the antibodies can be present for years before TSH leaves the reference range. Knowing that early changes how closely we watch it.

A few practical points. Biotin supplements can throw the assays off, so stop them for a couple of days before a draw. Draw in the morning, and anyone already on thyroid medication should take the dose after the blood is taken, not before.

Why symptoms cannot settle it

The symptoms of a low thyroid are fatigue, weight gain, feeling cold, dry skin, hair thinning, constipation and low mood. That is also a fair description of iron deficiency, sleep apnea, low testosterone, perimenopause and depression. A hypothetical 44 year old woman with every one of those symptoms could have any of them, or two at once. The thyroid panel does not get drawn alone. It sits next to a ferritin, a sleep history and the sex hormones, so the right thing gets treated.

Thyroid is one of the areas where "everything looks normal" often means one test was run. If nobody has looked beyond TSH for you, it's worth a conversation.

This article is educational and is not medical advice.

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