Changes in plasma concentrations of thyroxine and triiodothyronine in beef steers fed different levels of propylthiouracil

Effect of hypothyroidism on growth performance, carcass composition and meat quality of fat-tailed Lori-Bakhtiari lambs

The aim of the present study was to investigate the effects of induction hypothyroidism by propylthiouracil (PTU) on the growth performance and meat quality of fat-tailed Lori-Bakhtiari lambs. Eighteen Lori-Bakhtiari male lambs were randomly assigned to one of three groups (n = 6) and received daily treatments (gavage) consisting of 0 (Control: C), 10 (Low: L) or 20 (High: H) mg PTU/kg bodyweight/day for 60 days. PTU decreased plasma triiodothyronine and thyroxine concentration in both L and H (P < 0.0001). Lambs treated with PTU (L and H) had lower feed intake (P < 0.004), feed conversion efficiency (P < 0.003), and greater intramuscular fat than C lambs (P < 0.035). Meat from the L and H lambs had lower cooking loss and shear force, and also higher L* (lightness) than C lambs (P < 0.004, P < 0.015 and P < 0.025, respectively). The meat of H and L lambs was more tender than C lambs (P < 0.032). However, the meat of H lambs required fewer chews before swallowing than C lambs (P < 0.041). Generally, induction of mild hypothyroidism appeared to improve feed conversion efficiency and meat quality of lambs.

Effects of methylthiouracil treatment on the growth and moult of cashmere fibre in goats

The effect of long-term treatment of goats with methylthiouracil on the timing, amount and quality of secondary fibre (cashmere) growth and timing of cashmere moult in goats was investigated. From early June, groups of 10 Icelandic × Scottish feral goats were dosed orally each day, for a 15-month period, with 5 mg methylthiouracil per kg live weight in 30 ml water (treated; T) or with water only (control; C). Treatment with methylthiouracil resulted in a significant reduction (P < 0·05) in the proportion of active secondary hair follicles present during March. This was associated with a delayed onset of moult of cashmere in T compared with C goats at both the head (11 March v. 23 February; s.e. 3·33 days; P< 0·05) and mid side (27 March v. 26 February; s.e. 3·58 days; P < 0·001). There was no effect on the time of onset (C, 19 July; T, 19 July; s.e. 5·84 days) or cessation of cashmere fibre growth (C, 9 December; T, 8 December; s.e. 1·69 days) or the mean growth rate (C, 0·473 mm/day; T, 0·451 mm/day; s.e. 0·025) and fibre diameter (C, 16·9 μm; T, 15·4 jim; s.e. 0·266). Wlien present in the fleece, the mean weight and proportion of cashmere was higher in C than in T goats (P < 0·05). It is concluded that methylthiouracil treatment altered secondary follicle activity and the time of onset of the moult of cashmere and that these changes may be a result of reduced triiodothyronine production from thyroxine and associated secondary changes in profiles of insulin and IGF-1.

Effects of propylthiouracil and bromocryptine on serum concentrations of thyrotrophin and thyroid hormones in normal female horses

REASONS FOR PERFORMING STUDY

There exists a need for better diagnostic tests to characterise thyroid disease in horses. Currently available diagnostic tests fail to differentiate between thyroid gland disorders and thyroid abnormalities resulting from pituitary or hypothalamic problems.

OBJECTIVES

To evaluate the effects of treatment with propylthiouracil (PTU) and bromocryptine (BROM) on serum concentrations of triiodothyronine (T3), thyroxine (T4), reverse T3 (rT3) and equine thyroid-stimulating hormone (e-TSH, thyrotrophin) in mature horses.

METHODS

Healthy mature horses were treated using either PTU or BROM for 28 days. The effect of treatment on the thyroid axis was assessed by measuring T3, T4, rT3 and e-TSH before and at +14 and +28 days. The effect of PTU and BROM on the response of T3, T4, rT3 and e-TSH to thyrotrophin-release hormone (TRH) administration was also assessed before and at +14 and +28 days of treatment.

RESULTS

Treatment with PTU led to a significant reduction in serum concentrations of T3, T4 and rT3 on Day 28 and increase of e-TSH on Day 28 (P < 0.05). Treatment with BROM did not cause any measurable effect on serum concentrations of T3, T4, rT3 or e-TSH. The percentage increment by which serum concentration of T4, T3 and e-TSH increased following stimulation with TRH was decreased by treatment with PTU for 28 days (P < 0.05) but were not affected by treatment with BROM for 28 days.

CONCLUSIONS

These results suggest that 1) treatment with PTU may be used in horses as a model of primary hypothyroidism; 2) the use of BROM as a model of secondary hypothyroidism in horses is not supported; and 3) e-TSH assay deserves further investigation for the clinical diagnosis of thyroid axis dysfunction in horses.

POTENTIAL RELEVANCE

Propylthiouracil effectively causes primary hypothyroidism. There is substantial variability between horses with respect to their sensitivity to this substance when administered orally. Further studies pertaining to the characterisation of equine thyroid disorders are warranted and the use of both PTU for the experimental induction of primary hypothyroidism and e-TSH for the diagnostic characterisation of thyroid disorders in horses should be considered.

Type II and type III monodeiodinase activities in the skin of untreated and propylthiouracil-treated cashmere goats

The presence or absence of types I, II and III iodothyronine monodeiodinase enzymes (MDI, MDII and MDIII) and their levels of activity in the skin of goats, which were orally dosed for 60 days with 0, 1.1, 2.2, 4.4, 8.8, 17.5, or 35 mg(-1)kg liveweight day(-1)of the anti-thyroid, enzyme-inhibiting drug, propylthiouracil (PTU), were determined. Contrary to our earlier report that PTU did not influence skin MDII activity, the currect more thorough investigation (in terms of numbers of observations and the efficiency of the enzyme extraction procedure) indicated that doses of 1.1.to 17.5 mg kg(-1)liveweight induced a 2 to 3 fold increase (P = 0.01) in MDII activity. However, in three of the four goats treated with 35 mg kg(-1)group, activity was similar to that of control animals. There were no significant differences between treatments in MDIII activity but there was a trend towards lower levels of activity in the goats dosed with 17.5 and 35 mg kg(-1). It is concluded that there is significant MDII and MDIII activity in the skin of goats and that although there is none of the PTU -sensitive MDI enzyme, synthesis of T3 within the skin could nevertheless be modified through increases in MDII activity induced by lower T4 concentrations in the circulation caused by PTU. Changes in pattern of fibre moult induced by treatment with low doses of MD-inhibiting drugs may therefore be achieved through this effect. Since MDII and MDIII enzyme activity may be reduced by high doses of PTU, prolonged treatment with high doses of PTU may have adverse effects on skin tissue.

Roasted soybeans and an estrogenic growth promoter affect the thyroid status of beef steers

We investigated the interactive effects of a roasted soybean (RSB)-supplemented diet and an estrogen ear implant [Synovex-S (SYN), 20 mg estradiol benzoate + 200 mg progesterone] in young beef steers on measures of thyroid status before and after challenge injections of thyrotropin-releasing hormone (TRH) + growth hormone-releasing hormone (GHRH). Twenty steers (body weight 255 +/- 5 kg) were assigned to the following treatments: 1) no SYN and a soybean meal-supplemented diet, 2) no SYN and a RSB-supplemented diet, 3) plus SYN and soybean meal, and 4) plus SYN and RSB. Steers were individually fed 1.13 MJ metabolizable energy/kg metabolic body wt daily of an 18% protein diet. After a 5-wk growth period, all steers were challenged (intravenous injection) over a 3-wk period with three dose levels of a combination of TRH + GHRH (0.1 + 0.01, 1.0 + 0.1 and 2.5 + 0.25 microg/kg body wt). There were no dose by SYN or RSB interactions. Across dose levels, values for baseline plasma thyroid-stimulating hormone (TSH) were 0.37, 0.35, 0.61 and 0.33 microg/L for treatments 1, 2, 3 and 4, respectively (SYN, P < 0.07; RSB, P < 0.01; SYN x RSB, P < 0.03; SEM 0.06). Net areas under the response curve for TSH were 66.4, 51.3, 91.4 and 64.4 (microg/L) x min, respectively (RSB, P < 0.08; SEM 12.0). Similar treatment effects and/or numerical differences after challenge were noted for thyroxine (T4) but not triiodothyronine (T3). Baseline (2.22 vs. 2.00 microg/L, P < 0.02) and peak (3.07 vs. 2.03 microg/L, P < 0.03) T3 concentrations were less for steers fed RSB than for steers fed soybean meal. This study indicates that in young growing beef steers, SYN increases TSH release from the adenohypophysis and that the primary effect of RSB is reduced plasma T3, possibly through an effect on peripheral T4 deiodination.

Serum hormones during the estrous cycle and estrous behavior in heifers after administration of propylthiouracil and thyroxine

Two experiments were conducted to assess the characteristics of thyroid-stimulating hormone (TSH) during the estrous cycle and to examine the effects of experimentally altered thyroid status on estrous behavior and hormonal concentrations in heifers. In Exp. 1, normal profiles of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and TSH were examined on days 2, 12, and 19 of the estrous cycle (day 0 = estrus) in six Holstein heifers. Baseline concentrations of LH were 1.5- to 2.1-fold higher on day 2 (P = .07) and day 19 (P < .01) than on day 12. Pulse frequency of LH was higher (P < .05) on days 12 and 19 than on day 2. For FSH, baseline concentrations were similar on all 3 d, but pulse frequency was higher (P < .05) on day 19 than on day 2 or day 12. Overall and baseline concentrations of TSH were similar on all 3 d. Pulse frequency of TSH was higher (P < .05) on day 12 than on day 19, but both were similar to that on day 2. In Exp. 2, secretion of TSH in Holstein heifers was altered by administering (i.v.; one-half of the daily dose given twice daily) either phosphate-buffered saline (SAL; n = 6), propylthiouracil (PTU; 20 mg/100 kg body weight [BW]; n = 6), or T4 (1 mg/100 kg BW; n = 5) from day 4 until the next estrus. Duration of estrous cycles during and after treatments, percentages of heifers exhibiting estrus, BW, and average daily gain were unaffected by treatment. Concentrations of TSH were higher (P < .05) in the afternoon than in the morning and higher (P = .06) in PTU heifers than in T4 heifers. Treatment with T4 during the treatment estrous cycle and for up to 6 d into the next estrous cycle increased (P < .01) concentrations of T4 and T3 compared to SAL and PTU. Treatment with PTU during the treatment cycle increased concentrations of T4 (P < .05) and TSH (P = .06) compared to SAL. We conclude that TSH secretion was pulsatile, demonstrated a diurnal change, and varied inconsistently during the estrous cycle. Altering thyroid hormones and TSH secretion in heifers seemed to have no effect on estrous behavior or estrous cycles.

Relationships between the thyroid and somatotropic axes in steers. II: Effects of thyroid status on plasma concentrations of insulin-like growth factor I (IGF-I) and the IGF-I response to growth hormone

Three studies assessed the effect of thyroid status on regulation of plasma IGF-I in cattle. First, four Angus-Hereford steers (av wt 345 kg) were fed 4 mg/d propylthiouracil daily for 35 d. With continued feeding of PTU steers were sequentially injected with thyroxine (T4, 5 mg/d, IM for 5 d) followed by triiodothyronine (T3, 2 mg/d, IM for 5 d). An injection of bovine pituitary growth hormone (GH, 0.1 mg/kg, IM) was given to each steer on day 35 of PTU, day 5 of T4 and again on day 5 of T3. PTU alone increased plasma thyroid stimulating hormone (TSH), decreased plasma T4 and T3 but had no influence on IGF-I. T3, but not T4, lowered plasma TSH, IGF-I and the IGF-I response to GH (P < .05). Next, twelve bull calves (av wht 167 kg) were divided equally into two groups. A control group was injected daily for five d with buffered saline; the experimental group was concurrently treated with T3 (5 mg/d, sc) for five d. Beginning the sixth day, all calves were injected with GH (0.1 mg/kg, IM daily) for three d with the respective buffer or T3 treatments continuing. Plasma IGF-I was depressed 29% by T3. The incremental area under the three-d response curve was less (P < .03) in T3 cattle. A growth trial was conducted in which twenty-four Angus x Hereford steers were injected daily with T3 (2 mg/kg, bi-daily x 56 d) or implanted with Synovex-S (S) in a 2 x 2 factorial arrangement. Synovex increased empty body protein gain (EBPG) and plasma IGF-I 15.5 and 27.9% (P < .01), respectively; T3 decreased EBPG and plasma IGF-I 13.9 and 15.1% (P < .07), respectively, in steers which maintained suppression in plasma TSH. The data support the conclusion that elevated T3 decreases plasma IGF-I, in part, through a diminished GH-responsiveness and anabolic treatments such as S can reverse the effects of excess T3.

Relationships between the thyroid and somatotropic axes in steers. I: Effects of propylthiouracil-induced hypothyroidism on growth hormone, thyroid stimulating hormone and insulin-like growth factor I

The effects of propylthiouracil (PTU)-induced thyroid hormone imbalance on GH, TSH and IGF-I status in cattle were examined. In the first study, four crossbred steers (avg wt 350 kg) were fed a diet dressed with PTU (0, 1, 2 or 4 mg/kg/d BW) in a Latin square design with four 35-d periods. On day 29 in each period, steers were challenged with an intrajugular bolus of thyrotropin releasing hormone (TRH, 1.0 microgram/kg). Blood samples were obtained to assess the change in plasma GH and TSH as affected by PTU. Plasma IGF-I was measured from blood samples obtained before and after (every 6 hr for 24 hr) intramuscular injection of bovine GH (0.1 mg/kg, day 31). Doses of 1 and 2 mg/kg PTU increased plasma T4 (P < .01). At 4 mg/kg, PTU depressed T4 concentrations to 30% of control (P < .01). Plasma T3 linearly decreased with increasing doses of PTU (P < .01). Plasma TSH increased when PTU was fed at 4 mg/kg (P < .05) while the TSH response to TRH declined with increasing PTU (P < .02). Neither basal nor TRH-stimulated plasma concentration of GH was affected by PTU; the IGF-I response to GH tended to increase at the 1 and 2 mg/kg PTU (P < .01). In a second study 24 crossbred steers were fed PTU (1.5 mg/kg) for 119 d in a 2 x 2 factorial design with implantation of the steroid growth effector, Synovex-S (200 mg progesterone + 20 mg estradiol), as the other main effect. Basal plasma GH and IGF-I were not affected by PTU treatment. Synovex increased plasma concentration (P < .01) of IGF-I without an effect on plasma GH. The data suggest that mild changes in thyroid status associated with PTU affects regulation of T3, T4 and TSH more than GH or IGF-I in steers.

Influence of thyroid status regulation and Synovex-S implants on growth performance and tissue gain in beef steers

The separate and combined effects of Synovex-S (SYN) ear implants and thyroxine (T4)-5'-monodeiodinase inhibition (Trial 1) and T3 injection to create a mild elevation in circulating T3 concentrations (Trial 2) on BW gain and composition of gain were studied. Trial 1 used 24, 285-kg Angus steers in two experimental phases. Low-level feeding of propylthiouracil (PTU, 1.5 mg/kg BW daily) was used to achieve inhibition of T4-5'-monodeiodinase activity (TMA). Twelve steers received neither treatment (control) and 12 received SYN+PTU (hypothesized to maximize weight gain) from 0 to 56 d (phase 1) in a single factor treatment comparison. Subsequently, PTU was fed to six control steers and not fed to six of the original SYN+PTU steers from 56 to 175 d (phase 2) in a 2 x 2 arrangement of treatments. Trial 2 used 24, 302-kg Angus-Hereford steers. Treatments were without or with SYN and without or with sc injections of T3 in polyethylene glycol (2 micrograms/kg BW every 48 hr) in a 2 x 2 factorial arrangement of treatments. In both trials, all steers were individually fed a diet of a corn-based concentrate and silage mixture at an equal metabolizable energy intakes per unit of metabolic body weight (.25 Mcal/kg BW.75). Measurements of daily dry matter intakes, weekly BW, 28-d estimates of empty body components (measured by urea dilution), final TMA (trial 1) and plasma thyroid hormone concentrations were obtained. In both trials, SYN increased BW gain and protein accretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that bovine growth hormone treatment increases the rate of extrathyroidal 5'-monodeiodinase activity in cattle

This study examined the influence of bovine growth hormone (bGH) on liver and kidney thyroxine-5'-monodeiodinase activity (TMA) in growing beef cattle. In a preliminary trial (trial 1), tissue samples were obtained at slaughter from two placebo-injected and two bGH-injected (29.2 IU/day for 14 days before slaughter) Hereford heifers (398 kg avg slaughter wt), with one heifer on each treatment fed at either 1.0 or 2.0 times maintenance energy requirement. In a second trial, tissue and plasma samples were obtained from six placebo-injected and six bGH-injected (29.2 IU/day for 19 days) Hereford steers (331 kg avg slaughter wt) fed at 1.8 times maintenance energy requirement. In a third trial, liver tissue and plasma samples were obtained from 14 Angus X Hereford steers (270 kg avg wt) fed either an 8% protein, 1.96 Mcal-metabolizable energy/kg diet (8 steers) or a 14% protein, 2.67 Mcal-metabolizable energy/kg diet (6 steers) in association with acute administration of bGH. Half the steers in each group were given two injections per steer of either placebo or 37.8 IU bGH at 24-hr intervals and slaughtered 24 hr after the second injection. Units of tissue TMA in all trials were measured at slaughter; one unit defined as 1 ng T3 generated/hr/mg protein during incubation with T4 (1.3 fM). In trial 1, TMA in liver and kidney was 2.86 and 1.21 times greater, respectively, for bGH than for placebo treatments. In trial 2, units of TMA in liver homogenates were higher (P less than .05) for bGH (4.14) than for placebo (2.83) treatments and higher (P less than .02) in liver microsomal preparations for bGH (32.7) than for placebo (27.3) treatments. Units of TMA in kidney homogenates were also higher (P less than .10) for bGH (1.48) than for placebo (.87) treatments and higher (P less than .02) in kidney microsomal preparations for bGH (23.0) than for placebo (16.4) treatments. Following acute injection of bGH (trial 3), units of TMA in liver homogenates were higher (P less than .01) for bGH (2.5) than for placebo (1.8) treatments. Plasma T4 (70.0 vs 73.5 ng/ml) and T3 (1.08 vs 1.18 ng/ml) concentrations appeared slightly higher in bGH-treated steers (trial 2) and increased by 20 and .5 ng/ml, respectively, (P less than .01) within 6 to 12 h after acute injection of bGH (trial 3). Concurrently, plasma TSH concentration increased following acute GH injection (trial 3).(ABSTRACT TRUNCATED AT 400 WORDS)