Suppressed TSH Levels: Medical Journals

Purpose of this compilation

To show that a suppressed TSH (thyroid stimulating hormone) level in patients on treatment for hypothyroidism or hyperthyroidism doesn't necessarily indicate a non-euthyroid state.

A related compilation at this site is TSH Levels in Treated Versus Untreated People.

See also the links in the right column as well as in our T3 References and Desiccated Thyroid References sections.

A. Suppressed TSH, thyroid hormone levels, and euthyroidism
B. Suppressed TSH and bone metabolism
C. Suppressed TSH and cardiac effects

A. Suppressed TSH, thyroid hormone levels, and euthyroidism

1. "Thyroid function tests and hypothyroidism" (UK, 2003)

We have long taken the view that most hypothyroid patients are content with a dose of thyroxine that restores serum concentrations of thyroid stimulating hormone to the low normal range. However, some achieve a sense of wellbeing only when serum thyroid stimulating hormone is suppressed, when we take care to ensure that serum tri-iodothyronine is unequivocally normal.

Until valid evidence shows that such a policy is detrimental we will continue to treat patients holistically rather than insist on adherence to a biochemical definition of adequacy of thyroxine replacement.

2. "Suppression of Serum TSH by Graves' Ig: Evidence for a Functional Pituitary TSH Receptor" (Netherlands, 2001)

Antithyroid treatment for Graves' hyperthyroidism restores euthyroidism clinically within 1–2 months, but it is well known that TSH levels can remain suppressed for many months despite normal free T4 and T3 levels....We conclude that TSH receptor autoantibodies can directly suppress TSH levels independently of circulating thyroid hormone levels, suggesting a functioning pituitary TSH receptor....

...Such a mechanism may very well be responsible for the low TSH levels observed in otherwise euthyroid Graves' patients receiving antithyroid drug treatment.

3. "TSH as an index of L-thyroxine replacement and suppression therapy" (Ireland, 1992)

Suppressed TSH levels were associated with...normal FT4 levels in 62.5% [of the 90 clinically euthyroid patients receiving treatment with L-thyroxine].

4. "Thyroid stimulating hormone measurement by an ultrasensitive assay during thyroxine replacement: comparison with other tests of thyroid function" (UK, 1987)

Serum thyroid stimulating hormone (TSH) was measured using a highly sensitive enzyme-amplified immunoassay in 37 clinically euthyroid patients receiving thyroxine replacement therapy and compared with other biochemical tests of thyroid function....A suppressed serum TSH was found in 65% of patients with a normal serum total thyroxine.

5. "Clinical value of a sensitive immunoradiometric assay for TSH" (1985)

...our extended study here has revealed that a significant number of euthyroid patients with undetectable TSH (1.5% in our study) are likely to be found if TSH becomes the initial test for thyroid function. Thirty [out of 111] of the hypothyroid patients on thyroxine were found to have undetectable TSH, but only one showed clinical signs of thyrotoxicosis. Most of the patients, although having raised serum free T4, had serum free T3 levels within the euthyroid range or just slightly elevated.

6. "Therapy of primary hypothyroidism with L-triiodothyronine: discordant cardiac and pituitary responses" (1980)

...higher doses of L-T3 or substituting L-T4 therapy could suppress TSH secretion further without altering the other peripheral responses to thyroid hormone.

B. Suppressed TSH and bone metabolism

1. "Thyroid function tests and hypothyroidism" (UK, 2003)

The weakness of the meta-analysis, showing that thyroxine induced suppression of thyroid stimulating hormone led to reduced bone mineral density, was recognised by the authors themselves, who said that their design (cross sectional studies) was not appropriate because the many risk factors for bone loss do not allow correct matching of controls with cases.[footnote 4] This realistic assessment accords with the earlier views of Franklyn et al that thyroxine treatment alone does not represent a significant risk factor for loss of bone mineral density.[footnote 5]

2. "[Prolonged suppressive L-thyroxine therapy. Longitudinal study of the effect of LT4 on bone mineral density and bone metabolism markers in 71 patients]" [Article in French; abstract in English] (France, 1999)

Seventy-one patients (including 28 menopaused women) taking long-term L-T4 for thyroid carcinoma were divided into 3 groups according to their TSH level: low (TSH < 0.04 mlU/l), moderate (0.04 TSH < or = 0.10 mlU/l) and high (TSH > 0.10 mlU/l)....No lumbar or femoral osteopenia was observed in these patients taking L-thyroxin, even for those with complete TSH blockade.

3. "Suppressive doses of thyroxine do not accelerate age-related bone loss in late postmenopausal women" (Japan, 1995)

One group of patients was given suppressive doses of L-T4 (TSH <0.1 mU/L, n = 12) and the other group was given nonsuppressive doses of L-T4 (TSH > 0.1 mU/L, n = 12). There was no difference in bone metabolic markers and incidence of vertebral deformity between the groups....These prospective and cross-sectional data suggest that long-term levothyroxine therapy using suppressive doses has no significant adverse effects on bone.

4. "Suppressed TSH levels secondary to thyroxine replacement therapy are not associated with osteoporosis" (UK, 1993)

We set out to measure bone mineral densities in two groups of post-menopausal women receiving thyroxine replacement therapy (those with serum TSH levels persistently suppressed or non-suppressed) and to compare the results in both groups with those of the local control population....CONCLUSION: In this patient population, the reduction in bone mineral density due to thyroxine is small. It is unlikely to be of clinical significance and should not on its own be an indication for reduction of thyroxine dose in patients who are clinically euthyroid.

C. Suppressed TSH and cardiac effects

1. "Minimal Cardiac Effects in Asymptomatic Athyreotic Patients Chronically Treated with Thyrotropin-Suppressive Doses of L-Thyroxine" (US, 1997)

...in the absence of symptoms of thyrotoxicosis, patients treated with TSH-suppressive doses of L-T4 may be followed clinically without specific cardiac laboratory studies.