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 Low TSH normal T4 is called subclinical hyperthyroid which means incomplete hyperthyroidism, subclinical means a case of normal body function but blood test results show a little increase or decrease out the normal range,  hyperthyroid mean over production of thyroid hormone (T3 and T4).

What this topic taking about

• What does low TSH with normal thyroid hormone means?
• Low TSH normal T4 T3 causes and symptoms.
• What should you concern about?
• Does subclinical hyperthyroidism need treatment?
• Do low TSH and normal T4 cause death?

Note that:
Thyroid gland is a butterfly shaped gland, set in front of the trachea.
T3 means is the total concentration (free and bound) of tri-iodo thyronine hormone.
T4 means is the total concentration (free and bound) of thyroxine hormone.

T3 and T4 is two hormones produced by the thyroid gland after stimulation by the hormone TSH which secreted by the pituitary gland, both T3 and T4 are called thyroid hormone, thyroid hormone function is to initialize the metabolism, thus the body start processing substances circulating in the blood.

Free T3 is the free form of T3 hormone (unbound to a carrier protein).
Free T4 is the free or unbound form of T4 hormone.

The free form of the thyroid hormone is the active form and circulate without attaching to a carrier protein, FT3 and FT4 are better for diagnosis than T3 and T4 especially when blood proteins are high

The full story is:
The T3 and/or T4 is decreased in the blood (hypothyroid), the pituitary gland sensitive to the thyroid hormone level, the pituitary secrete the thyroid stimulating hormone (TSH) to stimulate the thyroid gland to produce more T3 and T4, TSH stops stimulation when T3 and/or T4 become within normal range, this process called homeostasis loop and means the human body adjust normal levels all the time.

TSH outside the reference range is likely to indicate that tri-iodothyronine and thyroxine levels are not normal for that particular person. Due to the log linear negative feedback relationship between serum thyroid hormone and TSH, even a small increase in serum thyroid hormone can suppress TSH secretion. Whether there is a difference between patients with a low, but detectable TSH versus patients with a fully suppressed TSH is unclear.

Subclinical hyperthyroidism definition:

• Subtle or unclear symptoms of hyperthyroidism
• Low or undetectable TSH level
• Normal or slightly increased T4,T3 with low TSH

Low TSH normal T4 symptoms and Signs:

Patients with low TSH and normal T4 T3 levels, whether due to exogenous causes or endogenous causes, were found to have a higher prevalence of heart palpitations, tremor, heat intolerance signs, sweating, nervousness, and anxiety, but many cases not overt symptoms of hyperthyroidism and thyroid gland work with autonomy function.

Examples for TSH low T4 normal:

1. Female non pregnant, 35 years old results:
Tsh less than 0.1 (<0.1 mU/l), Normal TSH level for adults is 0.3 – 4.5 mIU/ml, the result is lower than normal range.
FT3 2.5, normal FT3 for adults is 1.5 – 4,2 pg/ml, normal result.
FT4 1.2, normal FT4 for adults is 0.9 – 1.9 ng/dl, normal result.

2. Adult male, 40 years old, diagnosed for hyperthyroidism
First diagnostic results:
TSH 49 mIU/ml, T3 0.71 ng/dl, T4 4.1 ug/dl, results show high TSH and low T3 T4.
After period with treatment
Tsh less than 0.3, T3 1.5 ng/dl, T4 8.5 ug/dl, results show low TSH normal T3 T4
Normal T3 range for adults is 0.8 - 2.1 ng/dl, normal T4 range for adults is 4.6 – 12.5 ug/dl
3. Very low or undetectable TSH levels: TSH 0.005 or less, T3 1.8 ng/dl, T4 10.0 ug/dl, the results may increase after rechecking within a month and show overt hyperthyroidism symptoms, especially if ultrasound show small or large nodule.
Read: Normal TSH levels chart by age

Causes of low TSH normal T4 and T3 results:

Normal body function
Medical term is homeostasis:
Subclinical hyperthyroidism shows low TSH in a healthy person in general, body needs may cause short term changes in hormonal levels which appear as little abnormal results, considered normalized body balance and should go inside normal ranges soon.

1. 15% of whom have low TSH T4 normal is older patients.
2. Common low TSH normal T4 during pregnancy: Because B-hCG hormone has some thyroid stimulating activity, TSH may be suppressed during the first trimester.

Causes from outside the thyroid gland

Medical term is exogenous

Exogenous subclinical hyperthyroidism due to non-thyroidal illness, sometime called non-thyroidal illness syndrome (NTIS): thyroid allostasis in critical illness, tumors, uremia and starvation (TACITUS), non-thyroid illness syndrome can cause three euthyroid states, transient depression in TSH, T3, and T4, or low TSH, T3 and slightly high T4, or slightly high TSH and T3 T4 normal when recovery.

Iatrogenic subclinical hyperthyroidism: caused by using treatment drugs, the use of TSH suppressing medication other than thyroid hormone (for example, glucocorticoids, dopamine), using oral glucocorticoids (e.g. cortisone) is a known cause of suppression of TSH level without high T4.

Intentional suppressive thyroid hormone therapy, as in patients with thyroid cancer, thyroid nodules, or goiter.

In endogenous subclinical hyperthyroidism due to multinodular goiter, there seems to be a pattern of slow progression towards overt hyperthyroidism.

Overzealous thyroid hormone replacement therapy suppresses TSH production with normal thyroid hormone level.

Causes from inside the thyroid gland

Medical term is endogenous
Endogenous subclinical hyperthyroidism, the source of the mild excess of thyroid hormone in the blood circulation is the thyroid itself, as in Graves’ disease, multinodular goitre, solitary functioning thyroid nodules, or thyroiditis by viral infection.

Is low TSH normal T4 dangerous or cause cancer?

• High mortality rate is not clearly understood according to many scientific studies.
• Low TSH normal T4 is not clearly associated with developing cardiovascular disease, can induce cardiac dysfunction is less clear
• TSH low t4 normal may be developed to hyperthyroidism within 7-10 years especially in elders over 55, some other cases not became overtly hyperthyroid nor euthyroid during such period of time.
• Ultrasound findings (enlargement, nodules) predict, confirm, or decline abnormal thyroid integrity.
• Med-annual or annual checkup is recommended particularly in those where TSH remains persistently undetectable.
  Read: Recommended blood test for thyroid function

According to some studies:
Biondi et al found more atrial premature beats and a 20% higher mean heart rate in 20 relatively young patients with iatrogenic subclinical hyperthyroidism, compared with controls. In addition, echocardiographic measurements showed evidence of left ventricular hypertrophy in six of 20 patients.
All of whom had been treated with TSH suppressive doses of L-thyroxine for at least five years and all of whom complained of exertional dyspnoea. 

Both myocardial hypertrophy and diastolic dysfunction improved after treatment with β-blocking agents. Six months of individual tailoring of the TSH suppressive L-thyroxine dose aiming at a TSH of 0.1 mU/l was associated with normalization of echocardiographic parameters and an increase in maximal workload

Sgarbi et al treated 10 patients, median age 59 years, with endogenous subclinical hyperthyroidism due to a multinodular goitre (n = 5), a solitary functioning thyroid nodule (n = 2), or a diffuse goitre (n = 3) for six months with antithyroid therapy. After reaching euthyroidism they found a significant decrease in heart rate and in the number of atrial and ventricular premature beats. Echocardiography demonstrated a reduction of the left ventricular mass index, interventricular septum thickness, and left ventricular posterior wall thickness at diastole.

In conclusion, the majority of data suggest that thyroid hormone levels above the hypothalamic-pituitary set point (low TSH normal T4) have a negative effect on cardiac function and morphology. Furthermore, subclinical hyperthyroidism is associated with a significantly increased risk on atrial fibrillation.

What should you concern about?

Recheck the thyroid tests if it’s suspicious or the laboratory procedures not done correctly.